Or M1 Abrams. All variants served together in,let's say,1992.Then,I think,remaining 105mm were upgraded to 120mm
As far as I digged out informations, upgunning M1 is not an easy task, this is because there were fully developed two blocks of the M1:
Block 1 - M1 and M1IP
Block 2 - M1A1 and M1A2.
And now the interesting part, there are 3 versions of turret, I made my own typology to make things easier.
M1 use the Type 1 turret also known as "short turret", it have thinner front armor and there are probably other, minor differences. M1IP use Type 2 turret that is interim solution like the M1IP was when we consider whole tank. M1A1 and M1A2 use Type 3 turret, and then again even here are subtypes, because turrets of for example M1A2SEP have additional modifications.
Now the upgunning issue, if you want to upgun M1, you need to modernize it in to one of the Block II variants, which means a lot of cutting and welding work on the hull and... literally building new turret.
So even as Block I and Block II variants are part of the same vehicle family, in their basics they are also very different.
Actually the improvements that were made through they years expired or nearly expired vehicle SWaP-C characteristics, so the US Army is currently performing R&D program for ECP-1 (Engineering Change Proposal - 1) deep modernization program, that is intended to redesign or design a new all or nearly all internal components to make them more compact, lighter and power efficent, as well as to improve survivability by another armor modernization, some active protection solutions, and improve firepower by installing ammunition data link, which probably means replacing M256 with XM360E1/M360A1 that have data link as standard. But this is only list of currently accepted and needed improvements, by the end of this year or in 2014 there should be presented more specified list of improvements. Of course ECP-1 can be followed by ECP-2 and so on.
So actually besides a designation code the newer M1's have less and less in common with original design... but it seems to be normal for all weapons systems, when they receive improvements through their life cycle.
Edited by Damian, 03 January 2013 - 1331 PM.