I'm busy rebuilding my 10" to be mountable to an EQ6 but I did shorten it up in it's current Dob configuration to do prime focus DSLR photography. Mine was easy in that it uses struts so I just altered the top mount positions to lower the UTA by about 35mm.
But you're right, it does change the focal length and I am having to use an extender for some EPs to regain the focal length back. I actually use my 2x barlow with the lens unscrewed. I very rarely use it as a barlow anyway and I get twice the value
The rebuild is a Serrurier Truss design for lightness and it will be setup for Astrophotography focal length so I will have to still use an extender for some visual work. I still like looking at things as well.