@Mike Lavery_2 The disruptive upgrade recommended by HPE is a full reboot on both the IRF member switches which can be achieved simply by running the "reboot" command. This is going to reboot both the switches at the same time and thereby the time taken for the switches to come up post the reboot is the downtime you'd concede.
I would say a 1-minute downtime too less to anticipate(not being pessimistic). It actually depends on all the LPUs and their types. I would say we could give it a good 3 minutes at least.
I understand you are looking to minimize the downtime as much as possible to which the answer is what @parnassus precisely answered using ISSU which is for a compatible upgrade.
However, we have another approach for incompatible IRF upgrade viz, using MAD BFD. I'd encourage you to have a look at the below video explanation. I have tried this on a fixed port switch and has worked well in the past.
https://community.arubanetworks.com/t5/Video/Incompatible-upgrade-of-IRF-stack/ta-p/550292
Based on what I have observed I would strongly recommend a full-reboot explained in the first section unless you can test the second approach (using BFD MAD) before trying.
Hope this helps.