Couple of options that could work (although I haven't tried any tbh);

Give the rebased monster a few more wounds
Give the monster extra attacks
Increase the number generated for each monster (instead of D3 Minotaurs for example, D3 + 2, etc)

Personally I think I would go for the extra wounds AND extra numbers at the lower levels, but giving them both extra wounds and extra attacks when the monsters are individual ones, such as Giants, Hydras, etc, though you could balance this a bit by making them easier to hit with missile/magic attacks (ie +1 to hit or even +2 if very big such as Greater Daemons or Dragons)