Can the T-Mobile store turn my SIM card into a micro sim?

I'm buying an unlocked iPhone 4S from the Apple Store and it uses a micro SIM card

Depending upon the type of contract you have they may charge or do it for free, but all they do is to transfer your account settings from your old SIM to a new microSIM.

Note that although the transfer is often almost immediate, it can take up to around 24 hours for the network to register the transfer so you may be left unable to use your phone for a day. T-Mobile will not offer to cut your SIM down in size to meet microSIM size specifications.

Yes. They simply cut it to make it a micro-sim card.