question about switching gears?

Was talkin' to a guy... he says a 3.10 gear sucks for my demon set-up and he says i'll want like a 3.50 or a 3.90....now...i asked him if i could do a swap from a 3.10 to a 3.50 by just switching the ring and pinion, thinkin' it'd just be a swap and go...he said it has to be balanced, blah,blah,blah...is this all true?