In order to understand the differences between the various types of upper receivers, it’s important to know what was going on with the REST of the rifle at the same time.
In 1981, Colt developed a variation of the M16A1, adapted for the SS109/5.56mm NATO cartridge, and submitted it to the military trials as the M16A1E1. This rifle differed from the M16A1 by having the heavier barrel with faster 1:7 rifling, a different type rear sights (adjustable for both range and windage), round handguards instead of triangular ones, and by replacing the full-auto fire mode with the burst (limited to 3 rounds per trigger pull), to preserve the ammunition. It was officially adopted by US DoD as the “US Rifle, 5.56mm, M16A2” in 1982, which still is the primary infantry rifle for US Armed forces and a number of other armies and law enforcement organizations.
As you can see, the M16A2 upper receiver underwent some changes from the A1, namely the addition of the forward assist and the ability to adjust for elevation as well as windage on the rear sight.