As @eginnc says, best practice is to start from scratch, but...
(Following only applies to non swconfig -> DSA upgrades, we already know that's going to fail spectacularly, so don't do it.)
I personally want you to try to upgrade and keep your config, so we can find out what may go wrong. If you decide to try it, please report any issues (or success, too, that's valuable info) over on my owut thread, as the accumulation of such information helps me make upgrades cleaner and more robust. (Owut: OpenWrt Upgrade Tool)