This should answer most, if not all of your questions.
It’s important to remember, that you should rebase to the same DE to avoid issues. Although there shouldn’t be any data loss when rebasing, a backup of your files is recommended.
Edit: Forgot to write, that these principles apply to all atomic distros. But depending on the distro you’re using, the rebasing commands might be slightly different. So always look on the documentation of your distro.
As a workaround you probably could use a parcel forwarding service:
https://www.forward2me.com/
https://shipgerman.com/en/
https://www.no-border.eu/
I have no experience with these providers, but I’ve used similar services in the past and had good experiences for the most part.
But yeah, it really sucks that they aren’t able to ship to other EU countries, while other shops can.