Replacing some of the location mods in SRLE LOTD with some mods in REGS (e.g., replacing some of the Arthmoor Village mods with Expanded Towns and Cities) or adding some REGS location mods not in SRLE LOTD (e.g., the city mods) shouldn't require a lot of work in Conflict Resolution. This requires a little expertise with xEdit to look at the conflict resolution patches. Some of the Merges in SRLE LOTD (e.g., Blowing in the Wind, No Snow Under the Roof, Farmhouse Chimneys) would need to be redone to use some different location-mod-related patches.
The locations affected by REGS mods, including the location mods being reviewed and considered, can be seen here.
I think I am going to start by getting SRLE LOTD setup and then I will look at tackling the REGS portion as a copied profile in MO. Is there anyone else trying this already/ is this forum/thread a good place to ask for help when I get to that step?
I have never used the creation kit and only limited experience with loot and tes5edit. I don't know what xEdit is.