If you do not do it, you have only yourself to blame when something in it bites you 'unexpectedly'.
For the term of your contract, they _are_ required to provide the services specified in that contract, at the price specified. Unless there is something _in_the_contract_ that allows them to change things.
I can't speak for _your_ contract -- you *will* have to actually read all that boring stuff -- but such contracts usually provide that the carrier can change the service offerings, and maybe even the pricing, "whenever they want to", *BUT* if/when they do that, you have an 'escape' window, where you can 'refuse' the new terms. Depending on contract language, if you _do_ refuse the new terms when offered, that either *cancels* the contract, _without_penalty_, or they are obliged to continue the contracted services for the contracted period. Note well that the contracts are written such that if you _do_not_object_ then you are deemed to have agreed to the new terms, and they are binding on you.
You're not locked into long-term contracts. You *could* have bought service with _no_minimum_term_. Yes, all the major carriers do offer such contracts. If you did that, it would have been considerably more expensive, no "free phone", no free activation, or any other 'freebies'. You pay for it all, one way or another.
You have the right to get _what_you_paid_for_. As long as you are getting the services you contracted for, you have no basis for complaining about _who_ is delivering that service.
If you are _not_ getting the services you contracted for, then you _may_ have the basis for complaint. If however, the contract allows them to change the services delivered, and they notified you of the changes, and you did _not_ exercise your rights in a timely manner, you _have_ agreed to those changes, and you do not have recourse against them for your failure to act.
Note: this is just one reason why it *is* a good idea to read all those messy little details on the contract -- if not before you sign, at least sometime reasonably soon thereafter -- so you have some idea of what rights (limited though they may be) you *do* have. And so that you _can_ exercise those rights , in a "use it or lose it" situation.