unscrew fitting or cut/thread black iron pipe

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
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Rald wrote:

If it's natural gas, I'd think the chances of the pipe being over-torqued are less than for propane, because of the lower pressure. But it's probably well glued with pipe dope, which in my experience, with old water pipes anyway, needs a good application of heat (torch) to loosen. Heat the tee so it expands over the pipe if you can safely do so without blowing yourself up. Try it without heat first, with a piece of pipe on the wrench handle(s) for leverage.
Ken Grunke
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Rald wrote:

I would think under any case you can unscrew the existing pipe. Like the other poster said, it may take heat or some elbow grease, but unless the pipe has been in contact with the ground nearby, and is rusty or corroded, it should come loose and you should have no problems re-attaching that piece or a new piece to the existing tee. Just use gas grade teflon (yellow spool) and a bit a pipe dope when screwing it back in.
--
Gary Brady
Austin, TX
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