After a lot of searching, this has been an issue for a few years.. With NO stock solution or inbuilt solution within K2, it just seems crazy to me to change the address of every single article on you site.
The only solution is to use a commercial SEF component, but that just means if there is a component then it can be done, so why hasnt K2 done it, or provide an option..
I'm less and less inclined to use K2 for my main site until they have resolved this and I think its a main stumbling block for owners of established Joomla sites to convert to K2.