Incorrect. Modifications are perfectly permissible even for long periods of time as long as the firearm can be returned to the condition stated on the F1 or F4. The BATFE is very clear on that.
Sorry, but these guys have an official letter stating the same as I posted, just as official as those letters saying that you can't return a CCU to a pistol without paying the tax.
No, I don't have a copy I can scan in an post, but I have seen it.
Another case of .gov bureaucracy left hand not knowing what the right is doing, IMO.