I currently attend GSU part-time. I do not know of anyone who wanted to transfer from PT to FT that was not allowed to. This does not mean it does not happen, but I believe at least one student I know was below the curve overall and was still allowed to transfer. As far as I can tell, you just need to be in good standing academically to do so. People seem to transition between the programs regularly. There is no great division between the programs, so the change should be pretty easy. I can't even guarantee that they would say anything if you just started signing up for 15+ hours a semester.
The transition is not too bad. During your first year of PT (one fall and one spring semester), you take 19 hours, and you need 90 total to graduate. A normal 1L takes 30 hours per year, so you will be 11 credits behind at that point. 5-6 credits each summer can catch you up, and/or you can increase your load during the regular school year. The best way I saw students pick up 6 hours their first summer was our study abroad program in Europe.
I can't speak with any certainty as to the financial aid question. All I know is that I haven't heard of anyone having to drop out because they could not get loans. Also, our financial aid department is a nightmare to work with, but whenever I deal with them, I remind myself that I am going to the 4th cheapest law school in the nation (if you're not a Mormon), and that corners therefore need to get cut somewhere.