Hi Patrick,
Because new and old machines are same numbers you will have resources to temporarily run all guests only using new machines, so my safe and boring approach would be:
build new 3 node cluster connected to switches
setup up and test...
We just uploaded a 26.04 container template, the daily index refresh will pick it up for your PVE over the next 24h, to trigger one actively you can run pveam update as root on the PVE node and the new template should show up as available one.