os2firefox wrote:
Iirc, it's fine as long as you retain ownership of the original upper in the original configuration.
Correct, and I have verbally confirmed this with a BATFE agent over the phone (Yakima field office). I also confirmed that you can build an upper that is shorter than the one in your original configuration, as long as you retain that original upper.
I have read that you can make a permanent change to the original OAL that you declared on your form by sending a letter to BATFE notifying them of the change (no need for a new application & tax stamp), but I have not confirmed this. It makes sense; it's not like it's a dramatic change.