There are several changes to cards occurring with the release of Unfinity. You can find the official text of any card on Gatherer.

Copying Legendary Permanents

Many cards instruct you to copy a creature (or other permanent) and then tell you it isn't legendary "if that creature is legendary." We've decided to remove the last sentence from most such abilities. Although this is, in the most technical sense, a functional change to those cards, the only difference is making a permanent that isn't legendary continue to be not legendary. The gameplay with these cards should remain the same.

Landfall

Many cards with abilities that care about playing lands have the landfall ability word, while many others don't. Moving forward, we'll be including the landfall ability word on cards that match the function of existing cards with landfall abilities. Similarly, we are adding the landfall ability word to most existing cards with similar abilities. Adding the ability word doesn't represent a functional change to any card.

Surveil

Many cards, such as Curate, contain instructions in their printed text that are exactly the same as the surveil keyword action but don't use the word "surveil" in their text. Moving forward, we've decided that when we want to use that ability on a card, we will use surveil instead, and we've decided to update older cards with that text to match. This is a small functional change, and perhaps this will finally give my Dimir Spybug deck the respect it deserves (I'm not holding my breath, though). The changing cards are:

Un- Games

Several silver-bordered cards that referred to other silver-bordered cards, silver-bordered games, or similar received an update to include Un- cards from Unfinity that don't have a silver border.
These cards are:

Additionally, some cards in the Unfinity expansion received errata to clarify their function. Note that many of these are Un- cards and are not tournament legal.

Ambassador Blorpityblorpboop

Ambassador Blorpityblorpboop's original text was ambiguous about whether it was adding up all powers and all toughnesses of stickers separately or if it was combining powers and toughnesses into one number. Its new text clarifies this.

Ambassador Blorpityblorpboop's old text:
When Ambassador Blorpityblorpboop enters the battlefield, you get {TK}{TK}{TK}, then you may put a sticker on a nonland permanent you own.
At the beginning of each combat, you may have Ambassador Blorpityblorpboop's base power and toughness become equal to the total power and toughness of all stickers on permanents you control.

Ambassador Blorpityblorpboop's new text:
When Ambassador Blorpityblorpboop enters the battlefield, you get {TK}{TK}{TK}, then you may put a sticker on a nonland permanent you own.
At the beginning of each combat, you may have Ambassador Blorpityblorpboop's base power become equal to the total power of all stickers on permanents you control and its base toughness become equal to those stickers' total toughness.

Animate Object

The word token was omitted from this card's rules text when it was printed. The creature you're making is a token.

Animate Object's old text:
You get seven {TK}. Choose an inanimate object you own from outside the game. Put a power and toughness sticker on it. You may also put a name sticker, an art sticker, and/or an ability sticker on it. Put it onto the battlefield as a creature.

Animate Object's new text:
You get seven {TK}. Choose an inanimate object you own from outside the game. Put a power and toughness sticker on it. You may also put a name sticker, an art sticker, and/or an ability sticker on it. Put it onto the battlefield as a creature token.

Captain Rex Nebula

Captain Rex Nebula has received an update to its wording to clarify that the abilities it grants to the target nonland permanent last until end of turn. In addition, the Crash Land ability now causes the Vehicle to be sacrificed before it deals damage.

Captain Rex Nebula's old text:
At the beginning of combat on your turn, target nonland permanent you control becomes a Vehicle artifact until end of turn. Its base power and toughness are each equal to its mana value. It has crew 2 and "Crash Land — Whenever this Vehicle deals damage, roll a six-sided die. If the result is equal to this Vehicle's mana value, it deals that much damage to any target, then sacrifice this Vehicle."

Captain Rex Nebula's new text:
At the beginning of combat on your turn, choose target nonland permanent you control. Until end of turn, it becomes a Vehicle artifact with base power and toughness each equal to its mana value, and it gains crew 2 and "Crash Land — Whenever this Vehicle deals damage, roll a six-sided die. If the result is equal to this Vehicle's mana value, sacrifice this Vehicle, then it deals that much damage to any target."

Don't Try This at Home

Don't Try This at Home was printed with the word "whenever" as part of a replacement effect. However, "whenever" on cards indicates a triggered ability. It has been updated with the correct words.

Don't Try This at Home's old text:
Whenever a hot source you control would deal damage, it deals that much damage plus 1 instead. (Something is hot if it has hot things in its art, like fire or explosions.)
{1R}, Sacrifice Don't Try This at Home: It deals damage equal to twice the number of hot cards in your graveyard to target creature or planeswalker.

Don't Try This at Home's new text:
If a hot source you control would deal damage to a player or permanent, it deals that much damage plus 1 instead. (Something is hot if it has hot things in its art, like fire or explosions.)
{1}{R}, Sacrifice Don't Try This at Home: It deals damage equal to twice the number of hot cards in your graveyard to target creature or planeswalker.

Goblin Girder Gang

Goblin Girder Gang's text received a minor update so that its triggered ability checks both as it triggers and as it resolves to see if the result is stored on Goblin Girder Gang. For example, if 3 is not stored on Goblin Girder Gang and you roll two 3s, the ability will trigger twice. If you store the first 3 on it, the second ability won't resolve as it will see that 3 is now stored on Goblin Girder Gang.

Goblin Girder Gang's old text:
Reach
Whenever you roll a result not stored on Goblin Girder Gang, you may store that result on it. When you do, Goblin Girder Gang deals 1 damage to any target without a hat.
Goblin Girder Gang gets +1/+0 for each result stored on it.

Goblin Girder Gang's new text:
Reach
Whenever you roll a die, if the result isn't stored on Goblin Girder Gang, you may store that result on it. When you do, Goblin Girder Gang deals 1 damage to any target without a hat.
Goblin Girder Gang gets +1/+0 for each result stored on it.

Introduction
Comprehensive Rules Changes
Oracle Changes