If I have a Post Ban AR with an A2 stock and I want to change it to a Post Ban Tele-style stock, would that be considered Altering a Firearm, thus making my weapon illegal?

2)  How about swapping a 14.5 in with a permanently attached compensator, making it a legal 16 in barrel, with a regular 16 in barrel?

3) What about swapping a 16 in upper with a 20 inch upper? (of course both Post ban)

[newbie]

yes there all legal to do

as the post ban telestock doesnt collapse
the 14.5 with a permanent comp would make it fit the legal lenght

as far as federal law is consered
state law may vary
edited to add federal disclamer

One of the nice things about the AR is how much customization you can do to it. Replace the A2 stock with a post-ban tele-style stock...sure!

Buy a whole crapload of different uppers...sure!

As long as you put post-ban parts on your post-ban lower you should be OK.