[Bug-Candidate]:

[emrsrb]

[Bug-Candidate]:

[webthethird]

Hmm, I never thought of that issue.

In the generated contract DiffFuzzUpgrades.sol, could you try changing ^0.6.12 to >=0.6.12 and see if that compiles? If so, the fix should be fairly straightforward on my end.

Also, for what it's worth, soon I will be merging a PR which changes the flag for specifying the solc version to use from -v and --version to -V and --solc-version, so that the --version flag behaves the way most people would expect, printing the version of the tool itself. So for your case, when running the tool you would use:
diffusc v1_path v2_path --solc-version 0.6.12.