Switching Between Verizon and Project Fi

I've got a grandfathered Unlimited Data Plan on Verizon (which they can pry from my cold dead hands!) but I also have a T-Mobile SIM I use for my overseas trips (love the free, but slow, 2G roaming!). My Project Fi invite should be arriving in 1-2 weeks, and my plan was to cancel my T-Mobile account and use the Fi SIM for overseas roaming. My question is, if I upgrade the phone to the Project Fi-only LVY48C build, will I be able to switch between my Fi and Verizon SIMs at will, without any loss of functionality? Has anyone on Verizon tried this yet?

Thanks for any help y'all can provide!