You are making one gun, in one configuration (one OAL, BL and caliber). Upon your approval/stamp, You are free to swap out uppers, calibers, etc without any BATFE notifications.
On my SBR form 1's, I used to add in the additional detail section "to also be occationally used with other calibers; .22lr, 5.56, and 7.62x39, in barrel lengths of 4.5", 5.5", 7.5", 10.5", 11.5' and 14.5"... Its not required, not needed but it made me feel better and I thought it might help if ever stopped and the SBR was in a configuration that was obviously different that what was on the form.
I dont even bother with it anymore. I just list the initial details now...