ready to leave grandfathered unlimited plan

No, switching plans does not put you under contract. Provided you aren't now on a contract, you would be free to switch to another plan or another carrier at any time.

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@pdizzle555 wrote:

Hi - I am on a grandfathered unlimited plan with corp discount and am ready to leave... I was thinking to go to a metered plan since it is still cheaper than the current Unlimited Plus offering.  If I switch to that and realize month or so later I want to go to the Unlimited Plus

Yes, you can switch plans at any time.

or even change carriers - is that OK?  I guess I am trying to understand if I switch my plan do I enter any sort of contract ?

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No contracts any more (unless you still are in one), you only owe for the phone if you have next payments still.