ETA: this is actually one of my big concerns - selling my house and hating any new location and not being able to move back with the same super low cost situation I currently have. Relocation fails seem to be a common problem for many who move out of Calif
I don't want to derail thread, but I'd like to hear about more about this. Another thread, just continue derailing this one? :)
I had the same concern about relocating out of the U.S. for awhile.
As I've posted elsewhere, I've seen a lot of examples of geographic arbitrage go wrong, where people discover they can live in Thailand for ~10k USD/yr or less, build their internet business to support that, move there, and then get "stuck"--they can't afford to move back to the U.S., and feel trapped there.
I always wanted to live overseas in FIRE, but have enough of a stache that the option to come back wasn't even a question. In other words, even if we could FIRE cheaply overseas, we'd FIRE later, with more, so it wasn't our only option.
This would be similar to what Sparty's talking about: if you save just enough to relocate out of your high COL area for FIRE, but then decide you don't like that area as much, you may not have the means to go back (or would need to make income somehow, which may not be in your plans).
More specifically, if you have a nice living arrangement set up (small house you own outright that, if you sell, might be hard to purchase the equivalent later, or a landlord who's had you for years and never raises the rent cause you're a good tenant, so you're way under market, so if you came back, rent would be much higher, or whatever), and move away from that nice living arrangement, coming back might be difficult due to higher expenses in that category.
It's worth considering the various living arrangements you might have in FIRE, not just your primary plan.