You were talking about a smaller front sprocket weren't you?
In which case you would move the wheel back, ??though really I guess you should take a link out of the chain??
The Ninja 500R has a rear disk brake, so it should not need adjusting. I never adjust mine when I adjust the chain.
The EX500A (87-93) has a drum brake, so that may been adjusting.
Thought you had been looking for a 500R.
The main differences between the A and D are
A 16" front wheel
D 17" front wheel
D has bigger forks, but no drain plugs in forks
A has front disk on left
D has front disk on right
A has rear drum brake
D has rear disk
I'm pretty sure the rest is cosmetic. Knowing the differences is useful when looking at second hand stuff, you can easily tell which is which. I'm pretty sure the A has a silver painted frame, where as the D has a black one, but don't quote me on that one.