cavscout has nicely summed up a good number of qualities that would make a conversion difficult or unworkable.

It is the cut of a jacket that makes it a good candidate for a conversion or not.

Preferably you don't want to have to mess around with the lapels or collar. Best to look for one whose collar/lapels will work "as is"

If you can find a jacket that has pockets placed high enough to not look strange when you shorten the jacket you save yourself a lot of effort (plus you don't need to sew up the pockets, they can remain functional)

Cheers

Jamie