The amount of footage they have previously not used for a given season should allow them to take out the teams that weren't previously adapted without a serious detriment to their ability to make 40 eps.
Even so, it will not be the easiest thing to adapt. I don't think they would actually meet the previous teams as the Gokaigers will... more like just channel past powers. The real problem for Saban would come if they start mixing and maxing unadapted teams/villains with adapted teams/villains. I could see Saban pulling a MMPR Season 2 and make a *ton* of new footage.