That's essentially my question; when they happen does it mean I'm forced to use Marian troops? I don't want a reform in 150BC removing my ability to continue using Polybian troops, for example.

In a way I don't think there's much danger of that, I won't even hit the lower 45 province requirement by 110BC. I'll only have 30-something sticking to historical expansion.
I actually have the same concern regarding the up-coming Polybian reforms in my Romani campaign because, and you probably won't believe this, I actually don't want to lose access to Rorarii... I'm not kidding! I love those guys...