Basically, the only approach we've thought up is using a shared cloud folder (with something like OneDrive), which auto-syncs to a local folder on the SPC server.  (One client didn't find OneDrive reliable enough, so they set up an SFTP client on the SPC server to push/pull the files).

Awkward, but that's going to be the case with every cloud service.  The one thing I'm not particularly comfortable with for the SPC architecture, is it's a one server solution (not Sage server + user Terminal Server), so you really don't want users operating from the SPC server full time (with email...) and things like local file transfers for Sage input / output / links are indeed an issue.

What we are going to do is extract the memo tables and change the path to  the new path in the Sage Partner Cloud and then when we have moved over do an import to over write the path with the new path. 

What we are going to do is extract the memo tables and change the path to  the new path in the Sage Partner Cloud and then when we have moved over do an import to over write the path with the new path. 

There's also Link Maintenance's ability to update Memo attachment paths in bulk... but that is only practical when you don't have lots of sub-folders.

For actually transferring the files, here are the help files for a built in function for that:

Sage 100 - Transferring data using the Data Drive