We have defaults modules in Fedora. How to manage the defaults is documented here:
https://docs.fedoraproject.org/en-US/modularity/making-modules/managing-defaults/

However when I run on Fedora 30:

  dnf --releasever=31 --setopt=deltarpm=false --setopt=module_platform_id=platform:f31 --enablerepo=updates-testing distro-sync

I get likely issue about libgit2 module. The truth is that it is only one module which has more streams, is enabled by default, and changed the default between two Fedora releases.

Upgrades issues will likely happen in the future with the different module as well. E.g.,

Name                 Stream         Profiles                                Summary                                                               
gimp                 2.10 [d][e]    devel, default [d]                      GIMP

We have now by default gimp-2.10. Once there will be gimp-2.11 and the maintainer will change the platform default we will likely have broken dependencies or people will keep EOLed version of stream forever.

The behavior right now is that DNF will detect the default and create file /etc/dnf/modules.d/NAME.module and user has to manually change the stream despite the fact that the stream was selected automatically in past.
The problem is that the user - without any knowledge that modules exist - is expected to manually change the version of the stream of modules. But because he did not enable modules intentionally, he does not know that he should change something and he does not know what to change.

IMO this should be either changed automatically by DNF or we should have a tool which changes the version of the stream before or after upgrade to Fedora N+1.

And this definitely should be communicated to module maintainers how to handle the upgrade path.