Changing first part of building tree to any control part placed

[Anizer]

To start this off it's pretty easy to check out on your own.

So I was saving sub assembly's of my rocket and as usual when you create a new sub-assembly, your sub assembly disappears and you have to add it back if you need to use it. One time I saved a sub-assembly with the starting control piece (Command Pod Mk. 1) and when it saved it disappeared as expected but then I couldn't add it back to the sub-assembly. Since my initial control piece was missing, my building tree reset and I had to add a new one to be allowed to continue my rocket build. When I wanted to add my sub-assembly (with the Command Pod Mk.1) back, it had to be connected to the newly placed control piece and now i'm stuck with two command pods.

I feel that being able to change the initial piece of the building tree to any newly added command parts would make the ship assembly much more flexible.