GOG version of Serious Sam 4 etc

I have installed the GOG version of SS4. When I open WeMod
I am informed that the cheats for the GOG version are not available but
those for the Steam version are. Up until now, when I am in this
situation I have been able to point WeMod at the installed GOG
version and use the Steam cheats. I cannot do this now. There is
no way that I can find to point at the installed version. I follow
the instructions in Why can’t WeMod find my installed game?, but when
I select Install Game I am taken to the Steam page to download.
How can I now do what I have always done, ie point WeMod at the
GOG installation and use Steam cheats if GOG ones are not available.
I have checked with other games, eg The Technomancer, that I have
played before through WeMod using Steam cheats with no problems and the
same applies to these.
If this is no longer allowed, how can I change back to a WeMod version
that does allow this ? If I can, how can I stop it being ‘upgraded’
again ?