I feel your pain!
I would start by installing the ISE 2.4 URT (Upgrade Readiness Tool) on your Secondary PAN node and then seeing what it thinks. It analyses the state of your deployment and also gives predictions of how much time it will take to perform upgrade per node.
The URT is a good start as a sanity check of the database. Highly recommended, no matter which steps you follow next. You could either
1) Follow the upgrade path ... or
2) Rebuild each server from the .iso
I think the only clean approach is method 2. We do it all the time because of the misery of an upgrade procedure. In the VM world this is quite simple. In your case, it might require some work via the CIMC and vKVM etc - but ultimately it's possible.
Here are the high level steps
1) Make a config backup on your repository
2) Reboot your Secondary PAN using ISE 2.4 .iso - perform a fresh install of ISE 2.4
3) Restore the config backup onto this STANDALONE node
4) Make that node Primary, thus creating the foundation of your new ISE deployment.
5) Reboot your Secondary MnT using ISE 2.4 .iso - perform a fresh install of ISE 2.4
6) Register that MnT to your new PAN
7) Reboot a chosen PSN using ISE 2.4 .iso - perform a fresh install of ISE 2.4
8) Register that PSN to new 2.4 deployment.
9) Stop and test with a few NAS's to see if that PSN works as expected
10) Continue with rest of PSN's
11) Finally convert the last MnT node, and then if you are REALLY happy to proceed, then rebuild the remaining PAN node. Then all will be freshly built and registered to ISE 2.4 deployment
Keep in mind that patch 1 is not out yet. I suspect that ISE 2.4 is riddled with bugs - but as they say, "it depends" on what you're doing with ISE. Cisco message these days is that ISE 2.2 is the "stable release" - and they are aiming that ISE 2.4 will be the next stable release. They don't justify their comments. You have to read between the lines.
I am on 2.3 so I guess I pulled the short straw...
