Here are some options:
1. Buy a another G19 GenIII. Then keep or sell your other one.
2. Send your lower to Glock. For a price, they will trade your for a GenIII with a different serial#.
3. Buy GenIII lower separately. You'll have an extea lower, but will have a matching upper and lower if you should sell it later. You could also buy a Advantage Arms .22 kit for the extra lower.
Option 3 is the most expensive route. However, it gives you a GenIII lower for you tactical needs. It also gives you a complete .22 version for cheap plinking.