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Last year before 1L I applied for my gradPLUS loan right around this time. However because I am in the midst of the transfer process is it ok to wait to apply until I hear back from schools? (mainly because I'm not sure how much I will need depending on the school) or should I just apply right now as if I'm attending my current school and then reduce it later if need be?

I'd just apply for it now so that it's processed by the time you decide where you want to end up. I think there's a section where you get to add the school(s) to your application. Add every school that you've applied to, and/or considering going if accepted. The financial aid office of the school that you end up at would likely send you some sort of correspondence telling you how much federal loans you are entitled to. At that time, you can decide whether you want to take the full amount or adjust it.

now what happens if I don't hear back from any school before my current school's tuition is due? I can't imagine I'm the first person to be in that situation

Most school's tuition are due sometime in August, so hopefully you'd hear back before then. If worst comes to worst, you can pay your school's tuition and request a refund before you start the semester if you transfer.