It's (c).

I can confirm that forwarding to the new project does indeed work as one would hope. The id change is documented in the issue's history.

I'm keeping the old project alive for now. I'm not sure what will happen if I delete the OLD project and rename NEW to OLD. I expect that it will work, but I won't risk havoc to find out.

As a side note, I posted this on Reddit and got an answer in a few minutes, while there's dead silence here on SO for weeks. Sad 😢