Hmm, I can see that validity of using them still, considering there's models out there that load standard 7.62 NATO. Still, there are plenty of third world country militaries and militias that don't have an organized catalogue of their weapons. But on the original question, the M1911(A1 or not) is the oldest pistol in active service.
That's quite a shame that you'd have to hand it to the factory to change calibres, I like those weapons that are capable of switching right on the field and it would be useful especially for NATO troops. Imagine after sustained fighting over a period of time, you've run out your 5.56 and have nothing left but downed insurgents and their AKs with 7.62 Soviet. Switch of the barrel and whatever else then you're back in the fight. It's actually a situation SOF troops in Afghan suffer with quite a bit, they end up ditching their M4s or other standard issue and run with an old AK they've taken from a dead insurgent.
That was why Knights Armament developed the SR-47(Stoners Rifle AK47) which was practically an AR-15 loaded for 7.62x39mm. Unfortunately they only ever manufactured six for testing and one remains on display while the other five were handed to SOCOM, don't know if they still run it or not. The problem with that though was that it's meant to only fire 7.62 Soviet, it's nice to have a weapons system that can take calibres that your standardly issued and the calibre your enemy is running to make deep territory operations easier.
You know what's a new weapon I'm liking? The Polish MSBS Radon, a modular weapon that can switch from a traditional assault rifle to a bullpup configuration as well as change roles from assault rifle to carbine, sniper or IAR. I'm looking forward to when they implement those into the Polish Armed Forces and replace their Kbs Beryls. Personally, the Radon reminds me of the Masada ACR somewhat in traditional configuration and it doesn't help that they kitted it with Magpul accessories during its testing.
TL;DR Version:
M1911 is the oldest service pistol
Weapons that can switch calibres on the fly as a feature would be useful
KAC's SR-47 was a nice rifle but can't switch calibre
I want a Polish MSBS Radon