No worries - i too am new on the ICX bandwagon so glad i could help.Kudos for the assistance Craig. Everything you outlined above has worked like a charm. I was able to add 1/2/1 to the "new" VLANs and didn't need to re-add to dual-mode nor did I lose connection via SSH during the process. Setting up new downstream ports to my WAPs, switches and hypervisor was even easier.
Once I'm feeling brave enough I may dabble into L3 routing, but the UDM SE is working well enough right now as router-on-a-stick so no urgent need. Nice to know that the L3 is available if I grow into it in the future.
Remember if you move from the UDM pro for routing/filtering you will get more speed - BUT will have to learn how to write ACLs etc for the switch to perform the necessary filtering between VLANs etc - so thats another whole can of worms !
Once i get my ESXi hosts stable on the ICXs - that will be my next rabbit hole to go down !