Odly enough this very topic was covered today on a re run of ask this
old house ........

Rich T the plumber said if a tank is old its best to leave it be
unless its had regular service every couple years since frequently it
will cause either a instaneous or premature failure. he did a autopsey
on a old tank that failed 2 months after he flushed it. the bottom
half of the tank was totally filled with crud, the lower element had a
heavy crust of gunk, the anode rods nearly disappeared.

his pro opinion, old tanks shouldnt be disturbed if they havent had
regular maintence