As some sort of way to ease our trubbled fan minds...
I'll suggest that the Klingons aren't really using 200 year old ships, but merely sticking with a general design that works for them.
A diesel submarine from 50 years ago is fairly similar in appearance to a nuclear submarine today, at least in outward appearance. Yet we know that propulsion and weapons and systems are vastly different.
And a 50 year old aircraft carrier, say USS Midway, is again generally similar in appearance to a modern day nuclear carrier like USS Reagan.
I'd think the Klingons are more interested in strong workable warships that get the job done, rather than cutting edge innovation for the sake of innovation.
If Klingons can get a faster, stronger, more deadly vessel and it looks the same as the previous warship, what do they care? Plus we've since learned that Klingons are big on tradition and history so perhaps there's some honor is keeping an historic design.