I bought an AR 15 not all that long ago
I feel a little "cramped" holding it to my shoulder, I am 6'4" and long arms
To get it a little longer, could I have the buffer tube changed out, or would I need something else done to it?
I could be fairly accurate with it, but it is a bit uncomfortable
It does have an adjustable stock, but even pulled as far back as it goes, I feel a bit cramped

Sorry for the dumb newbie question
Other than this one, I have only had one other AR, it was an AR 10, and that thing was comfortable, but heavy
That's the reason I got rid of it

I’d bet you just need to try a different stock.

Post in the rifles section down below to get more responses

There are stock extensions available from most shooter's supply. You may find the fixed stock easier to lengthen and more comfortable.

Maybe an old style A2 (picture an old Vietnam era M16) style stock would work for you.

Vltor a5 receiver extension