@Ghoti , I would just try it, if I were you. Install Vivaldi. Follow @Pesala 's step by step guide to perform the import. The accounts are imported offline and you can test how all of it works offline. If everything looks like it should, you can enable the accounts and test some more.
I do not remember any particular file changes between Opera 9 and Opera 12, so things just might work out of the box.
@gmg Are there any plans to be able to import labels from M2? It's the one feature that is missing for me from the import.
I have quite a few labels in my M2 client. I could recreate them myself in M3, but it would take some time. But if importing labels is already on the roadmap, then I'd wait.
As far as I understand, Vivaldi's mail is not available to make this kind of request.
For this, we recommend apps that allow mass mailing / email-marketing campaigns.
@ayespy When I tried that yesterday, neither way worked, but when I tried it today, it worked! Really odd this stuff, but it works now, thanks for the help!