I purchased a S&W M4 when the first came out in mid 2000. I recently came into a M&P 15 Sport.

What are the differences between the early ones and the new Sports?

Thanks in advance for your assistance.

Without seeing the actual rifle you have, but here's what I've come up with. No forward assist, dust cover, carry handle, heat shield in the handguard, no cut in barrel for mounted grenade launcher. The trigger guard is built in the lower receiver, you cannot change it.

Plus 5R rifling and melonite treatment of the bore.

The M&P sports do not have the 1/8:5r barrel anymore. It's now 1/9. Unless you're referring to the "M4" the op is referring to.

I'm not sure what the OP is trying to compare... the early M&P 15s to the new Sport? Did S&W even make an AR back in 2000?

Without knowing what the OP has, we can't compare the two... We really don't even know which version of Sport he has.