I'm a bit confused as to your problem -- not sure what you mean by auto-load. Run on startup/boot?
Normally, you would not remove version 20. You would install version 21 and it would automatically port over your library settings and everything. Remove 20 later, both can stay on your machine). If you removed version 20 before installing version 21, you would restore a library backup file. Again, all should be ported over.
If you have done neither of the above and for some reason you can't or don't want to restore your version 20 library to 21. Then I would double check your import configuration to make sure that is what you want. Uncheck run import on empty libraries. And run a manual auto import.
If the autoimport option is checked, it should watch the paths for you in the background ... (bear in mind, this can be slow, or need a windows event for certain NAS's or external drives, but I don't think this is what you are referring to )