There is more to the M1s armor than the DU. The armor is a modular package that can be replaced by welding a new package onto the hull or turret. The DU is not the only change in those tanks. The shaped charge protection is not the same. There have been at least five iterations of the Burlington/Chobham armor since its inception. The tanks the US gives some countries, Iraq for example, likely are Desert Storm veterans with no DU in the armor or ammunition, older versions of the Burlington armor and a different fire control system. One advantage the downgraded M1s likely still retain is the kevlar spall lining that protects the interior of the tank. Russian vehicles generally do not have this which leads to even a non-penetrating hit creating fragments that can injure or damage people and ammunition. This is part of the reason the UK Army uses HESH rounds.