A Reddit data point indicates that product changing a Chase Freedom card to become Freedom Flex does not reset the $1,500 limit on the 5% categories. Reader BN also confirms this that you don’t need to activate the categories anew, and everything just carries over and continues where you left off toward the $1,500 limit.
People were wondering whether to wait for mid-cycle to do the product change and double up on the categories, but apparently that’s not possible.
Related: Reader Question: Is Chase Freedom Flex Product Change Considered A New Account?