A bit tongue in cheek, but with some truth - Abrams can be upgraded to Leo 2s / Leo 2s can be upgraded to Chally 2s
Chally 2s can be upgraded to T-90s, T90s to T-14s ...
No truth in that. The Abrams, Leopard, and Challenger were all part of the same original design. The US, Germany, and Britain went in together for a design that would be utilized by all 3 countries (MBT-70) but eventually went to individual designs with commonalities, that being the German gun and British Chobham armor, though the Germans eventually dropped the Chobham in favor of Burlington and the Brits went with a rifled main gun (L30A1).
The fundamental differences are the electronics packages within each vehicle and how each crew acquires and engages targets. The British and Germans rely heavily upon electronics for acquisition and engagement. This is problematic in the modern battlefield and something I was grateful that the Marine Corps did not buy into when I was a company commander. The electronics were minimal and primarily for data to improve first-hit percentages. The Army has computer laden vehicles that bogs down the TC and keeps him out of the fight. The M1A1 is simply better than the Leopard II and Challenger II, the T-90 ain't even in the mix.