In my experience you can fix most/many things via Bubble’s ability to run recursive workflows to modify or create entries in bulk. How easy or hard that is depends on the specifics of what you’re fixing.
100% read the book @lindsay_knowcode references. It explains what’s happening behind the scenes with Bubble, which then will help you clarify the right approach.
And if this is your first time building on Bubble, expect that you’ll do some things that you’ll want to go back and re-do.
With my app (heavily data driven) I was able to get it mostly right first go around. The one (and only) thing I’m still trying to optimize is a “list to list” search in a many-to-many relationship (Filter list from a list - #2 by ed727).