Current PT SUP here. I'm going to through a lot of info at you.
I started as a preloader and took a PT SUP position. Once you switch you get a pay bump (approx $3-4), guaranteed 27.5 hours a week, and you'll have to start paying for insurance. You also will lose your union status.
Now depending on your career goals being in management can benefit you if you have a degree and are willing to move. These two things can get you into FT SUP very quickly. If not you could be stuck as PT SUP for while. After about a year of chasing a FT job I found out that making the switch to management was a mistake. I don't have a degree, and I'm not in a position to move. So getting promoted internally would take years and even if there is an open slot for FT that doesn't mean the manager won't bring someone in from a different hub.
To get back into the hourly side you would have to be an outside hire. The outside hire is only eligible after 6 hourly employees are moved up to driver. So once again that could take years.
Luckily enough I'll finally be getting my chance on the road next week. I get to be the outside hire and get back into hourly position but I've had to wait 5 years for this moment. If I would have stayed hourly when I first got hired on I would have been a driver a year ago.
Hopefully that gives you a little insight.