iPhoto library

You can only convert each iPhoto Library individually to a new Photos Library, as described by Yer_Man. You can switch between Photos Libraries by quitting Photos, then double clicking the library you want to open.

I see, you have been reading this document "If Photos won't open a library that you already migrated". Photos will migrate each iPhoto Library only once. When it migrates a library, it changes the filename extension of the old "iPhoto Library.photolibrary" to "iPhoto Library.migratedphotolibrary". This tells it, that the library has been already migrated. If you need to migrate it again, because you did not keep the new Photos Library, change the filename extension of the iPhoto Library back to ".photolibrary" and you can migrate it again.

Are you trying to combine libraries or simply upgrade?

You can migrate to Photos very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.