there is no definite answer for this type of update, it highly depends on what's installed on your box. It is highly suspected the problems to solve could be numerous, and due to some very significant changes, the reinstall from scratch is best. If it's undesirable, then, you can just keep using it without update. If you would like to avoid reinstall, then there are 2 guides:
https://www.funtoo.org/Upgrade