Dnk:
It would probably be wisest to contact the local POCO, thier linemen/troublemen have a lot of experience in dealing with those things.
Even if the tubs are privately owned, they may be willing to take care of the tap change at no charge or maybe a modest fee.
There are a lot of factors involved, such as safely de-energizing them, ensuring no backfeed of any kind, dealing with the warm (or hot) oil, which may be PCB contaminated, not introducing any dirt, water or other foreign objects into the tank, how to actually operate the tap changer (Does it just turn, do you push it in and turn, pull it out and turn, does it take a special tool, is there a setscrew device or other locking device, etc.)
And are you prepared to deal with the possible violent failure of one or more of these if the tap changer or windings are bad? The POCO guys I know have told me horror stories from doing what they thought was to be a simple tap change.
If I were in your shoes, I wouldn't touch 'em. I don't have the necessary experience. I've read a lot about them, but I don't consider myself anywhere near qualified to deal with them!!
Good luck!!