Hypothetical Idea: M16 vs. HK 416, etc. replacement

I know there's a lot lot lot lot of talk and threads about the US military contemplating switching to a new weapons system, and from what I've been seeing, most of everything that these manufacturers are coming out with are AR-15 based derivatives modified for piston driven operation.

Wouldn't it be more economical for our armed forces to purchase a large number of just the upper receivers of the chosen gas piston upper rifle and keep the current lower receivers in circulation?

This way, we'd be able to quickly retrofit our service rifles to the standard of the day without having to retrain troops on an entirely new weapon system and at half the cost.

Here's a list of contenders up for military trials:
www.militarytimes.com...

[edit on 18-2-2009 by steve2071]