dnf --releasever=40 --setopt=module_platform_id=platform:f40 \

--enablerepo=updates-testing \
$(rpm -q fedora-repos-modular >/dev/null && echo --enablerepo=updates-testing-modular) \
--assumeno distro-sync

In addition to some problems that already have Bugzilla entries or such, and were already reported in this thread too, I got some problems with retired Rubygems:

$ LANG=C.UTF-8 sudo dnf --releasever=40 --enablerepo=updates-testing --assumeno distro-sync

Problem 3: package rubygem-byebug-11.1.3-5.fc39.x86_64 from @System requires libruby.so.3.2()(64bit), but none of the providers can be installed
  - ruby-libs-3.2.2-181.fc39.x86_64 from @System  does not belong to a distupgrade repository
  - problem with installed package rubygem-byebug-11.1.3-5.fc39.x86_64

Problem 4: package rubygem-shoulda-3.6.0-14.fc39.noarch from @System requires (rubygem(shoulda-context) >= 1.0 with rubygem(shoulda-context) < 2 with rubygem(shoulda-context) >= 1.0.1), but none of the providers can be installed
  - rubygem-shoulda-context-1.2.2-16.fc39.noarch from @System  does not belong to a distupgrade repository
  - problem with installed package rubygem-shoulda-3.6.0-14.fc39.noarch

Problem 6: cannot install both ruby-libs-3.3.0-4.fc40.x86_64 from fedora and ruby-libs-3.2.2-181.fc39.x86_64 from @System
  - package ruby-3.3.0-4.fc40.x86_64 from fedora requires ruby-libs(x86-64) = 3.3.0-4.fc40, but none of the providers can be installed
  - package ruby-3.3.0-4.fc40.x86_64 from fedora requires libruby.so.3.3()(64bit), but none of the providers can be installed
  - package rubygem-byebug-11.1.3-5.fc39.x86_64 from @System requires libruby.so.3.2()(64bit), but none of the providers can be installed
  - problem with installed package ruby-3.2.2-181.fc39.x86_64
  - package rubygem-pry-byebug-3.6.0-13.fc39.noarch from @System requires (rubygem(byebug) >= 11.0 with rubygem(byebug) < 12), but none of the providers can be installed
  - ruby-3.2.2-181.fc39.x86_64 from @System  does not belong to a distupgrade repository
  - problem with installed package rubygem-pry-byebug-3.6.0-13.fc39.noarch

I filed a PR for fedora-obsolete-packages for these [1].

[1]: https://src.fedoraproject.org/rpms/fedora-obsolete-packages/pull-request/86