Just reset the journal entry to draft state and then you can change the company if you are in a multi company environment.

If you can't reset the journal entry to draft that means either it's locked by an advisor access right or the journal is encrypted by hash in the journal configuration go to the journal in Accounting > Configuration > Journals and uncheck the Hash feature and for the lock try to open the lock from Accounting > Accounting > Lock Dates.

Happy to help :) an upvote will be awesome