Trying a different Leader with the same core, or building on a starter deck with booster cards? Compare the two lists and see the Leader change first, then every card that changed.
Deck A
Deck B

Your Leader sets the deck's colours, and every card in the deck has to share one of them. A Leader change is shown first, because a new colour usually means a large part of the 50 cards changes with it.
Starter decks make a common base: compare a starter deck with your list to see which cards you kept and which you replaced.
Each card is limited to four copies, so going from two to four of a key Character is one row with both counts.
Two starter decks led by Uta, released years apart: a handful of cards carry over as reprints, and the rest of the deck is new.
Open this comparisonYes. A Leader change is listed first, before the Characters, Events and Stages.
Yes, as long as both are One Piece Card Game decks. That is the quickest way to see how much of a core carries over.
Copy the changes as text for Discord or Reddit, or share the comparison link, which opens the same view.
More in the deck compare FAQ.