Ian,
I'm lucky, I guess, in not having a large collection of legacy documents. So I have less at risk.
As have you, I would guess, I've read several cases of Numbers 3.0 apparently messing up dates, or column references, or dropping charts when opening Numbers 2 documents.
As with transitions to different versions in the past for almost any software I've used, OS X or Windows, I've found it's wise to give imported documents a careful manual inspection "tune up" as it were, while keeping those backups!
Of course most of us don't like to do that. We want things to "just work."
And often things do seem to just work, but then we discover they don't quite, in small but important ways. Perhaps a little like my brain.🙂
SG