Armalite Series B lower with Series A upper???

[Xcount]

since it is impossible to find a Series B upper can I use the available Series A upper on my older lower?? I am trying to build a second upper instead of changing barrels when I want to change calibers.

Thanks for any firsthand experience with this.

 

[steve_podleski]

I have been 'no' but call armalite to make sure.