I read somewhere that you can send your gun (to Glock or ?) and have them replace the frame with one that has the newer rails, keeping your serial#.  Anyone ever do this?  This would give me justification to buy an M3 light for my G19, and use the light for other guns too.

I can't remember who the company was and I'm trying to decide if it would be worth the $.

I sent my 17 back to Glock for the newer frame with the rails. Cost was about $155 but I could not keep the same serial number. Glockmeister does the same service for $155 but I do not know if you can keep the same serial number.

New serial#?  It seems you would need to get the ATFE involved and fill out some forms.  How long did it take and did you have to use an FFL to receive the new frame?  Thanks!

No paper work to fill out and the gun shipped right back to my house. No FFL needed. I guess it was considered a repair.

At Glockmeister, a 4473 was filled out and my old frame was returned to me, much to my surprise. (I purchased a new complete lower)  I didn't know that was the protocol.  On the way home, it hit me, I could order the Advantage Arms kit and have a dedicated Glock in .22 rimfire.  That was an expensive day, but I am very happy and the wife now has a light on her Glock 17 nightstand gun.

A complete lower is not much more $$ than the bare frame.

While I was there, the Glock meister ultrasonic cleaned my Glock 21, installed new springs in it, and upgraded the ejector assy.  Free.  

I also bought some other trinkets for my other toys, like extended slide take down thingees and extended slide releases.  

I considered sending it to Glock, but the walk in, walk out, spend money, have new gun now was just too much to resist.  Glock was about a 6 week turnaround and you have to spend $30 to overnight it to them and probably the same to get it back.