Incompetent powder coaters

No. They will just chase the existing threads, not cut new ones. You'd need to ream out the old ones, fit a sleeve to the shell, and then thread afresh to change to english. Major pain in the arse.

I managed to cut a back to front english thread into a frame years ago, i was either doing it with the bike upside down or misinterpreted the left/right marking on the Park tool as sides of the bike rather than thread directions.
You'll probably struggle to get someone to do it for you though as it'd be far from the recommended way of doing things.
Luckily when i did it it was my own frame and i just ran a back to front bottom bracket in it.
Didn't take much effort to cut either, hence why i didn't notice i was arsing it up.