There is no direct switching between sync methods, e.g., between iCloud and Dropbox sync locations. The sync data isn’t shared between locations.

In DEVONthink: Control-click the sync location in DEVONthink’s Preferences > Sync and choose Clean Location. This will remove the sync data in that location. Enable the new sync location, adjusting the options for the location as desired, then sync a database. Enable other databases as needed.

In DEVONthink To Go: Tap the Edit link in DEVONthink To Go’s Settings > Sync: Locations. Add the new sync method, adjusting the options, and saving it.

If you are using a shallow sync, i.e., Download Files: On demand, and you are not syncing to a new location already synced by your Mac, you must leave the old sync location active. Tap the new location and sync a database. DEVONthink To Go will pull the contents from the old location to upload to the new sync location. If you are syncing to a new location to which you’ve already synced your Mac, the contents of the files should be in the sync location already.

If you are using a full sync, i.e., Download Files: Always, the content of the files in fully synced databases is already local to DEVONthink To Go, so tap the new location and sync a database. Repeat as needed.

After you have synced successfully, you can left-swipe the old sync location, and choose Clean to remove the sync data from the old location.