What they should really do; is honour legacy plans/prices.
Someone with a grandfathered $6.99/month premium plan will be allowed to stay on that rate until they cancel their plan. Suddenly this person's locked in for life because they know if htey cancel it; it'll cost $15.99 or whatever to sign up for it again.
It can be a risky move. T-Mobile tried something similar with their so-called "Price Lock" guarantee.