So I got a new Bushmaster 5.56/223 AR for Christmas and am starting to make a few new purchases to "enhance" it. Being new to the AR game, I have been researching how to replace the buttstock on my rifle. It seems as though the rifle I got has a pinned/ non adjustable stock. Is this the case with all AR's post law change? If I change to a magpul buttstock, do I have to re-pin it, or can I keep it adjustable??

You can keep it adjustable. Proceed at will

You might have a rifle buffer tube which isn't adjustable so might need a carbine buffer tube to go with your new stock.

Sell the Bushmaster!!!!!!!!!!!!

There are no laws in TX regarding a collapsible buttstock.

Sounds like it might be a fix A-2 style stock, i.e. looks like a normal rifle buttstock only it's black? You should be able to remove the stock from the buffer tube by way of the buttplate. If you want to change out the stock from a fixed to an adjustable you might have to change out the buffer tube but that's a quick fix.

If it is a rifle buffer tube and you change the buffer tube to carbine tube, you will also have to change the buffer and spring because the rifle spring and buffer is different than the carbine spring and buffer. You also need a castle nut and a plate. Also you should invest in a wrench to tighten the castle nut but you can do that with a punch if you're careful.