A few months ago I got a new passport. Just last week extended my annual Thai visa, & they transferred it to the new passport, no extra fee, no problem.

 

Now I need to do the same transfer with the ten-year China visa I got last year. Just wondering if anyone here has done this. When I got the visa the consulate was a madhouse and I paid enormous "fees" through an intermediary they had there to have my case expedited immediately. I wonder if this is de rigueur for transfer of visa as well.

You don't need to do this; the ten-year visa is still valid. Just present your old and new passport.