How did you create the resouce mailboxes in the first place? If you created them on-prem and moved them to O365, you should have all the relevant attributes populated on-prem. So make the changes there and wait for the sync.
If you have created them directly in O365 or "matched" them against existing objects, you are most likely missing some of the required attributes on-prem. In which case changing the on-prem attributes might or might not work :)