I note, that you prefer not to polute your new solution. That will reduce the subscription count, but if you get better leads, then that is fine.

One way would be to run a campaign on Linkedin or Facebook (depending on what you know, and where your audience is). There you can upload the single opt-in mailadresses, and campaign towards them. And then support that with a landing page with signup to your new environment.

You could also do "twin-identification" (at least on Facebook), where Facebook will match your audience with similar prospects based on demographics and interest.

You could also just import the 10.000 emailadresses in your new environment and give them a specific tag. Then you can easily delete them again, if they just continue to perform in the same way as they have always done.

There a many ways to do it, so good luck with your implementation. It would be a shame to just lose them.

If you want to, let's set up a call to discuss this further.

Best regards
Kenneth Wolstrup