Yeah, they are REALLY easy to alter (this applies to both the old shirts as well as the TI shirts from the looks of the seams in mine). On my spotless shirts, I altered them from being a size 16 down to about a 12.
Put it on inside out, figure out how much smaller you need to make it by placing pins along roughly where the red lines in the image below are, and then sew along the lines, removing the pins as you go. Do a straight stitch along the line, then cut, then do a zig-zag stitch on the edge to avoid fraying. For the spotted shirts I would personally avoid sewing the arm section due to the different sleeves, just sew the torso section.