Generally when it comes to an echo fighter, like 90-95% of their stuff is the same (moveset, character handling etc.). It’s going to be mostly the same.
Let’s just start with Mario and Dr Mario right, The reason why Mario and Dr Mario are so different is that their ultility is different. Dr Mario hits harder, is heavier, the forward air trajectory etc. make him different because you have to use him differently.
There’s a some other differences that are SO different enough that he’s technically not an echo fighter anymore because of how you use the tools that character has.
So for something like Princess Daisy and Princess Peach, you know Daisy DOES have some differences but it’s mostly...like thematic and cosmetic. The vast majority of the character is the same. Then they would be considered an echo fighter.
But I also wouldn’t put it out of the realm to see stuff like Chrom’s Up B. Although his moveset is very similar to Roy’s, Chrom’s Up B is not that. Yes, you will see some changes and it just depends per character so I am looking forward to people finding out what these changes are.