Still possible to upgrade ONE family plan to ONE Plus Family plan? (Note 10 BOGO)

I wanted to do the Note 10 BOGO that's currently happening but noticed I need a Plus plan for my current ONE plan or migrate to the Magenta Plus plan.

Is it still possible to upgrade my ONE basic family plan to the ONE Plus Family plan without changing my plan to Magenta?

Also, can existing customers receive the the 3rd line free on the Magenta plan or is that only for new customers?