Diy, of course. You just need some drawings to start, select a roof pitch.
The downside is your nice roofed space is going to be open to the elements for a period of time (contractor or diy, so you'd probably need to "bag" everything with plastic or good tarps). If you had time off work that you could merge with a nice window into the weather, some helper(s), and a pile of the correct materials already on-site, you could probably knock this off. The trusses are the long-lead item, the rest would likely be available locally.
Basically tear off shingles and roofing, tear off roof deck, tear down trusses or rafters to get down to the 4 walls. Working 1 wall at a time, remove double top-plates of the walls so you can sister in new 12' studs. I believe you should plan for center-blocking each stud cavity due to wall height > 10ft. Now you rebuild the double top-plates, set new trusses and resheath the roof. Get back to a layer of synthetic underlayment held down with orange cap nails and you are good for awhile to get everything else put back.
You might be able to re-use the trusses but they'd have to be taken down with due-care which takes longer. You might be able to get attic storage trusses depending on size and roof pitch which could be a driving reason to buy new.
I left out parts about permitting and inspections, those would have to be crossed also.