I'm sure that if someone has the older one's that they are fine.
But BCM wanted to do something about the rejection rate of the forgings and they did, this is when they changed and why they switched.
BCM recently started making their own upper receivers because their rejection rate was so high from their suppliers.
BCM found, that the uppers they purchased were not always in spec, and they decided to do something about it. They are now built on BCM plans and machined to those specs. BCM then took this a step further and made the inside diameter slightly smaller for a tighter fit.
There are better explanations out, but it was the first I found.
ETA- This is the one I was looking for:
Back story, BCM has been buying upper receivers out in the OEM market place for some time. They have discovered a problem with this. First is the fact that AR parts manufacturers will ship you whatever they feel like (meaning don't care if the part is in spec or not). BCM has had to reject upwards of 50% of the upper receivers they get in because they did not meet their specs and standards! Things like the Picatinny rail slots are off or not square and true or the inside dims are not right and your BCG is too loose or too tight, etc, etc..
Having had enough of this, BCM is having a machine shop make these uppers to their drawings. Lots of QC procedures are in place to ensure everything is correct (pics on this to come). There was one small change made from the TDP on these receivers. They made the Inside Diameter for barrel extension slightly undersized for a tighter fit to the barrel. By doing this, you will improve the accuracy of your AR.