About armor in M1 series.
Basic M1, M1IP (Improved Performance) and basic M1A1 got so called Burlington armor (sometimes incorrectly and popular called Chobham), also M1IP have new turret, let's call it "long" turret type 1, the frontal armor thickness was increased from 650mm LOS to 875mm LOS. The M1A1 have "long" turret type 2 with more upgrades like new blow-off panels, probably also this was first M1 model with upgraded CWS, or maybe that was M1IP? I don't remember.
(estimated protection for M1 Burlington armor is 400mm RHAe vs. KE and 700-800mm RHAe vs. CE over turret front, for M1IP and M1A1 it is 450-500mm RHAe vs. KE and 800-990mm RHAe vs. CE)
Then in 1987 US.Army has fielded M1A1HA (Heavy Armor) with major armor upgrade, 1st. generation armor with depleted Uranium layers (it gives estimated protection about 660-680mm RHAe vs. KE and 1080-1320mm RHAe vs. CE over turret front), it was infact probably based on older Burlington armor, just new materials like DU were added. In early 90's two new variants were fielded in US.Army and U.S.M.C., first was M1A1HC (Heavy Common) with 2nd. generation armor with depleted Uranium layers (880-900mm RHAe vs. KE and 1310-1620mm RHAe vs. CE over turret front), I think it was new armor, not based on Burlington but as I can assumpt have more common to British Dorchester armor known from FV4034 Challenger 2, as we know US and UK have very good relaitionship so cooperation in designing new technologies is more than certain.

Of course there were more upgrades in this variant, like redesigned ammo racks in bustle ammo magazine, new digital engine software for better fuel economy etc. second variant was M1A1HA+ (Heavy Armor Advanced), as I believe it is only small upgrade for older tanks, in other words armor packadge from M1A1HC. But probably in later years all M1A1's in active service were upgraded to M1A1HC standard.
M1A1HC is also base for M1A2.
The later upgrades for M1A1 series were M1A1D (Digital) upgrade to make M1A1's more compatibile with M1A2's, but there were only 2 Bn. armed with these tanks, no more reported as I know.
In early 2000's almost all M1A1's in US.Army and ARNG active sercie are upgraded to M1A1AIM (Abrams Integrated Management) v.1, this means zero hour-mile condition, major repairs, sometimes instalation of FBCB2-BFT system, etc.
Now all these M1A1's are upgraded to M1A1AIM v.2 or as it is other designation M1A1SA (Situational Awerness). This means 3rd. generation armor with depleted Uranium layers (940-960mm RHAe vs. KE and 1320-1620mm RHAe vs. CE over turret front), new FCS, TIGER AGT-1500C engine, FBCB2-BFT as standard equipment, TIS for CWS, TIP and other upgrades.
Also all M1A2's, oldest M1A1's and all (?) M1IP's will be upgraded to M1A2SEP (System Enchancement Packadge) v.1 and M1A2SEP v.2.
This means 3rd. generation armor with depleted Uranium layers, FBCB2-EPLRS, new FCS, new hoffman batteries, upgraded AC system, and other upgrades.
But, there is probability that M1IP's turret are not converabale to new standard, so only hulls are upgraded and turret's are new produced one's from low rate production.
Australia bought modified M1A1SA's, there are no DU in armor, we can assumpt it is somewhere on M1A1HA+/HC and M1A2 level.
Arab countries recieve export "monkey" models of M1A1 and M1A2 variants, we can assumpt that both export variants have same level of protection. These tanks are newer than M1A1HA from 1987, we can assumpt that their protection is similiar to M1A1HA or somewhere between M1A1HA and newer 90's variants.
About downgrading ammo. Well it is not in pure meaning downgraded, but have less armor piercieng capabilities than US currently used APFSDS ammo. More KEW-A1 and KEW-A2 ammo have nothing common to US used M829, M829A1 and currently used M829A2 and M829A3. US use DU alloy penetrators, KEW-A1 and KEW-A2 are tungsten alloy.
Some estimations:
US/Egyptian KEW-A2 120mm tungsten 660mm at 2km;
US M829A3 120mm DU 765mm at 2km (2003) (Russian estimate 795mm);
US M829A2 120mm DU 730mm at 2km (1994);
US M829A1 120mm DU 610mm at 2km (1991) (Russian estimate 700mm);
US M829 120mm DU 552mm at 2km (1987).
You mean what US newest ammo is not in use? M829A2 and M829A3 that seen combat in Iraq?
Or you mean AKE (Advanced Kinetic Energy) sometimes called as M829E4 that when will be fielded recievie M829A4 designation or other.
There is also AMP (Advanced Multi-Purpose) that will supercede M830A1 MPAT.
And US never allow anybody to use their curently based ammo. But ok, you can believe in any PDF, but foreign policy is foreign policy, and for US currently ally can be in future a foe, so there is better to not gave them the best equipment that US have... the other hand is also Israel, and Israel surely don't wan't US give any Arab country the best ammo, the best tank armor packadge etc. More You can be certain, this never happen that US give anybody their best technology.
