If you're an existing customer you will want to move to the newer plans, whether New Classic or Value. There is no catch at all, no contract renewal, you can take advantage of the unlimited services, at lower rates. If you currently have a contract, you just need to fulfill it to its end date, and that's that.
If you have lines on your account that are still under contract AND lines that are out of contract, bridge to value lowers the cost of the no-contract lines while you wait for the contracts on the other lines to end. So, when that contract does end, you move over to value with no migration fee, and it helps you get Value pricing for the no-contract lines while on a classic plan.
wait the 18 months and then change to value.. therefore, not paying the migration fee. you could always see whether or not paying the migration fee upfront helps you in the longer run with savings over that 18 month time frame (or however long you are in your contract)
And this is why T-Mobile is doing away with that. As an existing Classic customer you can switch to a new Classic plan that offer lower rates with no penalty (no contract renewal either). When you reach a certain tenure you can get your monthly bill even lower by switching to the new Value plans.