In this case, I think I’d lean towards the “it doesn’t matter in the grand scheme of things.”
RLGL and AI FTP Detection will still see the “real” work you did. In terms of PLs, it won’t be reflected – but perhaps you could note that the equivalent workout you completed was about a Threshold 3.0-4.0 workout so that you can jump into the right Threshold session for next time. 
Otherwise… there’s not really an “easy” way to change the workout from Mount Field to something else without going through a bunch of other steps (which, in my opinion, wouldn’t be worth the effort of doing).