ATmega328PB is not a drop-in replacement for ATmega328 variants, but a new device. However, the functions are backward compatible with the existing ATmega328 functions. Existing code for these devices will work in the new devices without changing existing configuration or enabling new functions.
In a world where there are ample 8-bit microcontrollers, I can understand why Arduino does not produce a -328PM model; there is unlikely market drivers to make such a board a profit for Arduino who is constantly in competition with clone manufactory. Perhaps a re-spin of the UNO formfactor due to a large commercial purchase (school, colleges, etc.) would provide a break-even opportunity.
SOAPBOX OPINION:
I read numerous forum posts and a very large numbed of posters are using clones primarily because they are less costly. Of course they are, they are copies or near replicas. Some companies do produce original "compatible" products BUT do they provide a costly forum to discuss problems and issues? Hosting is not free and with increase scrutiny on energy consumption due to global warming, electrical & cooling costs are going to increase.
Hobbyist have always been cost-conscious and prone to buy discounted in order to play at the hobby. Everyone understands to a degree buying clones - but, may I suggest that all design efforts and published/public projects use an Original Arduino. When it comes to replicating a working template for friends and family, cheap clones are inevitably prudent. Your initial purchase of all models of interest from Arduino will provide two important components:
- Increase sales by Arduino to defer operating costs;
- Focus on issues in these forums to concern regarding "real" hardware and not issues from 3rd party clone sloppiness or lack of Quality Assurance.
Regards,
Ray