Is it possible to upgrade to UW a not UW mk2?
No, you can only upgrade a +drive. No UW upgrades as far as i know.
Has anyone just ordered the +drive and installed it themselves?
If you can get hands on a +drive it should quite doable to install it yourself. ‘Sadly’ Elektron does not sell the the +drive seperately. Your only chance is to either send it in or find someone that sells an MD for parts or something like that.