Yep, pretty much what everyone else has said. No cross training. But you might be able to transfer completely.
I started off as a seasonal SF hire. After the holidays, my hours got cut from 32/wk to 8/wk. I couldn't stand my ETL either, so when I heard of an opening, I asked to transfer. I had to interview for the position with 2 APS's, the ETL-AP, and a random ETL. I only had a high school diploma, but was attending college for an AS in Criminology. I got the position, transfered departments, and I was allowed to stay in my store. That was years ago. Since then, we've had a few people switch from SF/store side to AP and stay in the location, but only 2 have been able to switch from AP to store side and stay at the location. This is usually frowned upon because you'd know camera placements and confidential information. And even though I was a TPS, I was occasionally allowed to walk around the SF and assist guests.
To the best of my knowledge, a high school diploma/GED is the only requirement. College experience (graduate or currently attending) is preferred. You must be good with computers/technology (writing reports, operating camera system, etc). Apprehension certification will be instructed by your district/group team. You should know that much of the job consists of standing for hours with only your 15 mins/lunch breaks. Above all, you have to be someone your ETL team trusts and respects since you have moderately high responsibilities. If you have more questions, I'd suggest you talk to your direct ETL to let them know you're interested and ask if it's okay if you schedule a meeting with your ETL-AP (I didn't do this and my ETL got angry when he found out I was interviewing for the position).