Tak L. unfortunately updating the figma kits automatically does not exist today. Your method seems to be one method, the other would be the opposite of that which is to duplicate the latest, make style updates to the style guide and turn it into a figma library and then include that new figma library into your existing projects.
One question I have is - what is the biggest motivating factor for why you want to migrate your existing figma kits from 2.4 to 2.8?