New shock on 98 Homegrown URT

I have a 1998 Schwinn Homegrown Factory URT. I have built it from the frame up, with Manitou Minute 3:00, SRAM X.0 shifter/der., XT Hollowtech II cranks, and lots of Ti. I still have the stock Rock Shox Deluxe rear shock and that is 6+ years old, and would like to upgrade to a newer rear shock. The frame is 17", and is just a tad small. My first question is if I add a longer rear shock to essentially lengthen the seat tube, what other effects would this have. Also, since it is a single pivot point, can I change the rear shock length without affecting the suspensions performance? I'm leaning towards a cane creek air shock or a manitou swinger.