eDirectory Upgrade standalone

I am upgrading OES 2018.3 to OES 23.4 using the ISO 'physical media'. (Can't go to 24.1 as the Customer Centre configuration doesn't work - need an activation code - which is not available with the OpenText doing maintenance).

Finished installing packages and is up to eDirectory Upgrade.

It requires an IP address of an eDirectory server that holds a replica of the context of the server.

This server is standalone. There is no other replica server.

I entered the IP address of the current server (which validates successfully) but this generates an error saying that the replica server IP must be a remote address.

I can understand that this would be the case for a New installation; however, this is an Upgrade. 

Is there a way around this point, or do I need to revert?

An very similar query has been asked before, but those replying did not read the OP and no solution was offered.

 eDirectory upgrade during OES 11 to OES 2015 upgrade 

I could create a VM that just has eDirectory to provide a replica. Would this server need to run 2018.3 or 23.4? Is that critical?

I assume that this would still mean rolling back my current upgrade so that the new server can replicate the eDirectory.