Most FBOs will get you to fill in a 'knowledge quiz' and then send you up with one of their instructors and just give you differences training (i.e. 152 -> 172) and do a few T n Gs. This is especially if you are going from high wing to low wing aircraft where perception is very different. Once they are happy you know what you are doing they will cut you loose in it.
Nothing to stop you going to a complex as soon as you have done your PPL. The 100hrs requirement may be something stipulated by certain clubs before they will let you solo one.
Julian.