Merge Buddypress Users With Free Level of M2

I see a couple threads that mention bulk adding of existing users into an M2 level, but my installation of M2 does not have that "all user" option. I have thousands upon thousands of members and adding them into the free level of M2 one at a time is just not an option.

What's the best way to merge all existing WP/BP users into the free level of M2?

Additionally, can members of one tier "upgrade" to a paid tier at any time?

Thanks in advance.