Do you need a new sim card when you change your number?

When you change your number, do you have to do it in person? Can you do it over the phone? Does it cost money? I'm asking about T Mobile phones...how long does it take, and do you need a new SIM card??????Do you need a new sim card when you change your number?
Under normal circumstances a network will only change your number when u are bieng bombarded with prank or malicious unwanted calls. In that even the network would request suitable documentation from the local police etc etc. However if u just want a new number the best way is to just purchase a new sim card. Im unsure of ur location but here in the UK u can enter any of the t-mobile stores and obtain a new card for free so long as u put 拢5 of credit on it. that would really be the easiest soloution go get a new sim card, just dont forget to use up all ur remaining credit on ur old sim as nomally any credit CANNOT be transferred :)