I was in a similar situation a while back when I upgraded my rig to an i9-10900k. My Digi 003c was running smoothly on Windows 10, and I was worried about the same thing. What I found out is that your
windows 10 key can actually be used to activate Windows 11 as long as it's a legitimate copy. So, you won't need to buy a new key for Windows 11. As for the Digi 003c, it should work on Windows 11 too, but you might need to check for updated drivers or compatibility issues. I'd suggest doing a bit more research and maybe contacting Digi support to confirm, but you might not need to replace your sound card after all.