Migrate to Win 11 before or after changing HDD ?

If you replacing HDD to SSD, then you would need to make clean install of Windows 11.

You can backup all of your important data to another drive/USB/OneDrive first.

Once backup done, you can insert SSD to disk slot in your laptop/PC. 

On the SSD, you can perform clean install of Windows 11 latest version using this community guide:

https://learn.microsoft.com/en-us/answers/questions/2338451/how-to-make-clean-install-of-windows-11-article

Let us know if this helps!