Much like the Moto X, Moto G, DROID Maxx, DROID Mini and DROID Ultra - also the iPhones - the HTC One (M8) uses a nano SIM. Which is the smallest SIM card available right now. So if you're buying the HTC One (M8) as an upgrade, you're probably going to need to either head out to your carrier store and grab a new SIM card once your HTC One is in hand, or you'll need a SIM cutter. Which I think is the better choice since it'll also come with a SIM adapter, so if you want to switch back to a phone with a micro SIM card, you're all set.
It's actually very easy to cut down your SIM card. I had to buy one for my Moto X late last year, and it was really simple to do. Just place your SIM card in the tray, push it into the slot in the SIM card cutter and push down. Then your basically all done. It's really just that simple.
However, if you don't want to cut down your SIM card, you can always head to your wireless carrier store and get a new one. Typically they'll give you one for free, but you'll want to wait until your HTC One (M8) is in hand before doing that because your existing SIM card will no longer work. You could also order another one from the likes of StraightTalk, or Simple Mobile. They both have nano sized SIM cards available and are MVNO's of AT&T. Basically they run on AT&T's network and are much cheaper.
Just a PSA to let everyone know before their HTC One arrives, that it's going to need a nano SIM card and not a micro. If anyone has any questions about cutting down their SIM card, be sure to let us know in the comments below.