If you're within contract:
As long as you're not getting a new device or contract, you may change to ThePLAN with a monthly recurring fee (MRF) commitment that's on the same level as your previous plan, or upgrade to a higher level, without having to pay for any preterm fees.
Just remember to keep your account clear of any outstanding balances before doing this.
If you downgrade your plan, you'll be paying a preterm fee computed in this manner:
Handset price - Handset cashout x Remaining months in
24 months your contract
Once the pretermination fee has been settled, you can start creating your new plan and even avail of a new handset.
If you're no longer under contract:
Yes, you can change your plan anytime if you're already outside of your contract.
If you're out of contract, you may switch to ThePLAN anytime and there are no preterm fees to pay.