Migration assistant replace user account greyed out

Can't reply to the original thread because there hasn't been any activity there for a while so I'll start a new thread because I have found a solution that works for me. Seemed like a good idea to share my findings here and hopefully give others some insight into this issue.

Original thread: https://discussions.apple.com/thread/8127693

Short description of the problem: When using the migration assistant to migrate a backup onto a freshly installed computer the option 'Replace user' is greyed out.

In my case my backup was of a Macbook running MacOS Catalina and the new Macbook Pro was running High Sierra. When trying to migrate my backup with the new Macbook Pro I ran into the problem of the greyed out 'Replace user' option in the migration assistant.

I then decided to update the new Macbook Pro to Mojave and when I ran the migration assistant again the option wasn't greyed out anymore and I could finish the migration without any further issues.

I guess in my case the problem was that the OS on the backup was too new for OS on my new Macbook Pro.

Catalina and High Sierra are not compatible apparently. Updating the Macbook Pro to Mojave did the trick, meaning that Catalina and Mojave are compatible.

Apparently there are users that run into this problem when trying to migrate two systems that run the same OS. For them this solution will not work obviously.