The standard procedure in this case is to create a local replica copy, set the db encryption to nothing, then use an agent to Refresh all the documents therein.
The Refresh is not a simple F9 - view refresh, it is a full-blown @command([ToolsRefreshAllDocs]) agent that is run from the All Documents view and which will take time in proportion to the number and size of the documents it is to work on.
Once that is done, all encryption should be removed from the local replica. You can then replace the server copy with that one (with the Administrators help, of course).
Pascal.