Well this seems to be the inclusive list for Milwaukee products that currently won't work with the 12.0
**Due to the physical size increase of the M18™ REDLITHIUM™ HIGH OUTPUT™ HD12.0, it does not fit the following tools prior to a 6/1/2018 manufacturing date without applicable service parts: 2787-20, 2788-20, 2771-20, 2122-20, 2130-20, 2135-20, 2361-20, 2145-20, 2146-20, 2360-20. Milwaukee is committed to protecting users’ investments, offering free service upgrades for the listed products to any customer that purchases an HD12.0 battery pack. Users may visit eService at
https://www.milwaukeetool.com/eService for available service upgrades.
So if you buy a 12.0 they will upgrade your product somehow
https://toolcraze.net/milwaukee-m18-12-0-ah-6-0-ah-battery-uses-21700-cells/