I am have just created a new Snapmirror between to clusters - version 9.5 P6.
The Relationship Type is XDP and it is an Asynchronous Version-Flexible Mirror. This all appears to be normal.
I also have many other Snapmirrors that were created years ago. They were most likely created on Ontap version 8.3 (or perhaps earlier). These Snapmirrors have a Relationship Type of DP and are just simple Mirrors.
I am in the process of migrating all of our Volumes to a new SVM onto new storage and will be recreating the Snapmirrors the cluster where I have just created the XDP Snapmirror, again on a new SVM.
What differences are there between the XDP and DP Snapmirrors and the Asynchronous Version-Flexible Mirror compared to the Mirror type. I assume that I wouldn't want to downgrade the mirror from XDP to DP if this is even possible?
For the specific question, which is a very good question, I will try to explain:
Traditionally, there has been a fundamental difference between DP & XDP(vault):
"DP" are block-based and uses BRE (Block Replication Engine)
"All the rest of it", (file-based) i.e XDP, SnapVault and version independent SnapMirror uses LRE (Logical Replication Engine)
BRE (suited for disaster type of scenario) = Tracks each "block" and is unaware of file-system structure.
LRE (suited for archival type of scenario) = Tracks the data "logically" and is based on the file-system structure. So when the resync is attempted, BRE cannot find any snapshots that hold block based references to the data so it reports that there are no common snapshots since the block based engine cannot understand the logical tracking. If you attempt to convert a relationship, which was originally an XDP relationship to a DP relationship, and then attempt to update it, the snapmirror will report 'un-supported error'. However, an ORIGINAL DP relationship which is later converted to an XDP relationship = Can be converted back to a DP relationship, provided the source and destination volumes still have Snapshots with the same instance and version UUIDs. This answers your query "I assume that I wouldn't want to downgrade the mirror from XDP to DP if this is even possible?", possible but XDP advantages outweighs DP.
"DP" mode is no longer applicable starting ONTAP 9.3 = With improvements in performance, and significant benefits of version-flexible SnapMirror outweighs the slight advantage in replication throughput obtained with version-dependent mode. For this reason, starting with ONTAP 9.3, "XDP" mode has been made the new default!