Many packages use the same spec file across all the Fedora branches (since there is very minimal difference), so proposing a forceful change to all spec files to use the SPDX identifiers immediately would break that workflow and necessitate having either different spec files for branches or conditionalizing the license filed (which I don't even know is possible with RPM).

IMO it would be better to do the change in 2 stages separated by 2-ish releases. The first change allows the use of the SPDX identifiers in F37+ alongside the use of the old specifiers. Packagers could then change their spec files to use the SPDX IDs for the branches that support it at their leisure, or continue using the single spec for all branches approach. Then a mass conversion to use only the SPDX identifiers could be done once F37 becomes the oldest supported branch - allowing the use of a single spec file using the SPDX ids for all branches at that time.

You are forgetting about Epel. And waiting for phasing out Epel... that takes time.

I see nothing wrong on

%if 0%{fedora} > 37
License: Bar                                                                                                                   
%else
License: Foo
%endif

And yes, it works.

I agree, though, with two stages. First allow SPDX - beside old short name. In second step, convert everything remaining.

Miroslav