Well in that case you might need to reach out to Support who may have an official workaround or fix. Ive seen this in the past in Epicor 10.
In the past when I break the Conversion WorkBench I use SQL and UPDATE the Conversion thats stuck on the Ice.CnvProgs table, simply marking it as ProgStatus = ‘COMPLETE’ - or - blank so I can run it again. But that’s me, don’t want to encourage SQL Updates, not knowing your technical skillset.