Its always wise to remove any and all compromised wood when doing any repair if you can...sometimes it isn't critical if it is not structural ( usually if plywood is in a fiberglass boat it is for structural/stiffening reasons) and other means then can be used as a quick fix temporary fix..but if you take the effort and time to replace it you will feel better about it and so will the future surveyor and buyer.
As far as how big of a job it is ..that's hard to say depending on your boats configuration...the actual repair is usually way easier then the tearing apart and reassembling of everything to get at it.
"Go Simple...Go Large"
Relationships are everything to me..everything else in life are just tools to enhance them.
The purchase price of a boat is just the admittance fee to the dance...you still have to spend money on the girl...so court one with something going for her with pleasing and desirable character traits others desire as well... or you could find yourself in a disillusioned relationship contemplating an expensive divorce.