Do you have hardware appliances or VM's?
If VM's, then you can build out 6 new ISE 2.3 VM's and leave them at the 'setup' prompt (assuming that you are also required to re-use the same IP addresses from the existing ISE deployment). But, if you are free to create 6 new ISE nodes with new IP addresses then it's even better - just build all 6 nodes and leave them as Standalone mode. This might be possible if you are using a load balancer. All of your NAS's send Radius/WebAuth to a VIP and therefore the NAS does not have a hard coded ISE IP address.
Create another interim ISE VM with ISE 2.0 (or 2.1... whatever the interim version needs to be from 1.4 -> 2.x upgrade). In that new ISE 2.0/2.1 VM, restore the config backup from ISE 1.4. Check that the config looks sane.
Make a config backup of that 2.0/2.1 system. Import that backup into ISE 2.3 - now you have a new PAN node for your ISE 2.3 deployment! Register all the other node personas like MnT, PSN etc. Once you new deployment is built, you can start adding in some PSN's into the load balancer pool (if you have a load balancer). If no load balancer, then change a NAS config to use the new ISE PSN.
Basically, if you have a VM deployment AND IP address of ISE nodes is not hard coded in your NAS's, then you have the luxury of running the old and the new ISE deployments in parallel - and you can decide when to migrate to the new platform.
If you plan to upgrade ISE using the upgrade method (as opposed to restoring a config backup) then you really need to test this. And even if your tests pass, there is no guarantee that it will work on your production system. Having been burnt in the past, I would never upgrade a system ever again.
