You can send your pistol directly to GLOCK, and they will upgrade it. You can also send the entire assembled upper, but not the lower, and they will replace the lower with the new model for a larger fee. If you do that you have an extra lower you can use for other things, an Advantage Arms .22 upper for example. Or you can cut the handle down to fit the G19 or G26 mag, and switch out the frames when you need extra concealability. You can ship the upper by USPS, as it is not a firearm, but you will need an FFL holder to receive it back for you.
I believe (but may be wrong) that they no longer match the serial number for you. BATF was worried about the possibility of two guns floating around with the same S#.
As to which models have generations, The G26/G27 have only ever been available with finger grips and without accessory rails. The G34/35 have only been available with 3rd gen frames, but you can put then on older G17/G22 frames. I know the G17 was available in all three at various times, and I know the G19, G22, and G23 came in at least gen 2 and 3.