Scaling classic adventures: Is there a downside?

[S'mon]

Wow; all great input. Thank you! S'mon: I think I may use your giants leveled up by 2-3, just because I'd like to run G1-3 at levels 11-15 rather than 8-12. (I don't use XP, so advancement is easy for me to control.)

If you're running G1 at 11th level, I recommend the MM3 hill giants, they start at 11th level and are very nicely designed!
G2 at 13th level might work with 12th level elite frost giants or 14th level standard -probably the latter for 'hordes of giants'; then levelling down the Monster Vault frost giant is probably best. Also look at MM3 frost giants. Good thing is that as brutes they are easy to hit.
G3 at 15th for fire giants: soldiers so high ACs but you could I think use the MV fire giant as-is if your group is good at hitting things.

So potentially it looks to me that you could get away with only changing the frost giants for G2, and only by a couple levels.