Yes, AK-47 was already replaced in Soviet times. Actually, twice.
And yes, the problem of caliber is an interesting one. It's not only an armor penetration problem - small calibers are somewhat lacking in stopping power and lighter bullet is easier affected by atmospheric conditions. On the other hand, less weight and more flat traectory... I'm not sure, but the actual solution may be some intermediate caliber. Of course, there are many reasons to not changing it!