Ok. Cool.
I didn't try this, as I don't know if paypal's IPN do follow redirects...but it would be interesting to know:
If you still own the old domain, put a redirect-rule in your htaccess redirecting to the new one while keeping the all URL parameters.
If it's only one user, contacting him is probably easiest.
You can edit subscription and change the validity manually, if it didn't expire, or no events are needed on your site.