Hey long shot I love the idea of using the BCG and charging handle from the other upper that will save some money for sure . Would you suggest a go/no go gauge just to be safe ? I would check that out first .
As far as practice building . Not sure thats a good idea . you can and most likely wind up spending more money then needed and have a bunch of extra parts laying around you decided not to use . Trust me I learnd the hard way on this point . I have almost enough parts to build a complete AR just sitting here . All I really need is an upper receiver and barrel . Hum ??? I should totally do that .
I like the idea if you really just want to learn and get the feel of things . I would just upgrade the one you have . That way you will have a very nice gun and not just another average upper that you will most likely put more money it to later and have even more parts laying around