My recommendation is to carefully evaluate that decision. What is it you expect to gain from replacing them?
Not to say that the GMs don't have their bad points but they also have their good points. The primary good point is that they are already there! A secondary good point is that they will accept waste that a lot of heads will not. Plus everything about the head pump is easily rebuildable. The only bad point about them to me is that they are noisy. But I periodically consider replacing them for that reason. Oddly enough, the Admiral is the one who always says, "They work great, leave them alone!"