Yes, it's been done before (KNX, KKOB are two examples that come to mind) but the barrier is high.
>>>>>>
The only way I see this working is if a candidate to be the replaced station, r, crashes and burns and starts having such dismal financial performance that there's a compulsion to do something about it. Then the replacement could take place and there would be less risk by starting from a lowered base of revenue. There could also be externalities such as a sports team or university insisting on a full-market FM station for coverage. Until the time that these things might happen, KOA is on a couple of translators with peripheral coverage in most circumstances, and, for FM-exclusive listeners, it's not a full-market signal, which also isn't exactly the path to long-term success. And, yes, there may be AM-curious listeners but that cohort is dwindling. Under present circumstances, KOA seems to be stuck. (Edited this paragraph in hopes of making it clearer.)