There is no "PD19 with the 365 subscription." You'd edit with one or the other.
Right now, PD365 is based on PD20. You can open any project made with PD19 in PD20/365 but once you save it, it cannot be opened again in PD19.
It's always been that way with new versions of PD. Next year, PD365 will be based on PD21 and you won't be able to open any of those projects if you have PD20.
I don't know what happens to subscription content once you stop the subscription. That's normally separate from the PD app and it's possible you'd still be able to use it if you cancel the subscription, but I wouldn't plan on it.
You can ask CL customer service and see what they say.
https://membership.cyberlink.com/support/index.html?action=contactUs&action2=productContactUs