You'd be crazy to leave for Southern for SIC Guppy time if you're about to upgrade on the RJ.
A 737 type without PIC is as worthless as a 777 type without PIC. Ask any of the guys stuck there with thousands of hours of that shiny 777 time, all in the right seat.
Yeah, upgrades are happening fast on the 737, but it's unlikely you'll be offered a shot soon, with only 400 hours PIC from your CFI days.
If you were stuck at your current regional with no upgrade in sight, it would be a different story, but since you're about to upgrade, STAY PUT. Get your turbine PIC.