Cannot Open BPMC Models

I am trying to open my old BPMC models. They will not open.

I assume it is because the permissions on the accounts have changed. When I created the original models, it was possible to collaborate on models. But now it is not possible to collaborate with a professional account. But I don't want a business account, paying for two users. It is just me now.

How can I open the .bpmc models with a professional account?

Do I have to sign up for a month of business to get my models back?