Erik,
If you enable attachments prior to creating the replica the attachments will sync up when you perform your synchronization.
If you enable attachments after the replica is created the best option is likely to unregister your replica and then register using the 'register existing data only' option.
The register existing option would look something like this:
a) Copy delete the feature class on the relative replica and copy the class (with attachments) to the relative replica. Ensure you have globalIDs before copying).
b) Sync the existing replica to make sure each side has all pending changes.
c) Create a new replica that includes the changed class with attachments enabled using the “Register Existing data only” option. This will take only a few seconds to complete as the data already exists on both sides.
d) Assuming no additional edits had been done b/w steps b & c, unregister the old replica. (if edits had been done, just make sure to sync the old replica before unregistering)
e) Now move forward with the new replica.
