Legacy features and legacy pricing are entirely separate.
If your group is old enough, your Basic group will retain legacy features (and if you're paying for Premium anyway, this point is kinda moot).
To keep legacy pricing, you have to remain on the same plan. If you ever allow the group to downgrade -- or even switch between monthly/yearly billing -- you will find yourself stuck with the new pricing structure from that point forward.
Hope this helps,
Bruce
