This is a t-shirt alteration I've been doing for years! At my old job I always had to wear Half Price Books t-shirts to work, and they normally only came in boxy lady sizes or really large unisex sizes - but always with a very tight crew neck. I hate crew necks!
This trick takes only a couple minutes to do, and I've even included a tip about what to do with your sleeves if they're too long or too tight. :D
Please note that this only works on jersey/knits - and no hemming is needed because the fabric won't fray. (Please see the last step for more info!) It might roll a bit after washing, though! You can also do this trick with a piece of chalk, some pins and some nice scissors, too. The rotary cutter is just easier.