I've modified a larger assembly with 2020 and now found out that my colleagues only have the 2018 version of SolidWorks. I briefly studied my options and saw a mention of SP5 that can be used for transferring models to the previous version. So, will I be able to transfer my 2020 version model to the 2019 version first and onwards to the 2018 version?
Nope, that only works one version back (with limited functionality). Will need to use a neutral file format to transfer (Parasolid, .X_T) and then your colleagues will only get dumb solids.