I want to put in a tee for a new drip line and flexible gas connector
when I replace my water heater. It is currently connected with piping
all the way to the water heater. The problem is that there is not a
union fitting where I would like one to be.

In order to make the modification, I have to try to unscrew a section
of 30 year old Black Iron Pipe attached to an old 'T' OR I have to cut
the line after the 'T', rent a threader, cut threads, and install a
union. (after shutting gas and purging with air of course)

What are my chances of gently unscrewing the pipe out of the old 'T'
fitting and then installing a new section of piping into that old 'T'.
I have experience with new galvanized piping, but I have not tried to
re-use 35 yr old fittings. If the pipe was originally fitted to the
'T' without over-torquing, is there still a good chance that it can be
re-used OR would it be wise to just plan on renting the threader and
leaving the old fitting alone?

Rald