Threaded pipe systems are built from the bottom up. Taking a section out of the middle requires some special considerations. Depending on how old the pipe is, taking it apart might create problems...the weakest point may break.
You'd need to cut out a section of the pipe, then unscrew the stub from the joint. To keep stresses down, you MUST use two wrenches - one to hold the part you are keeping stationary so it can't turn or twist, then the other one to actually unscrew the stub.
Since when you install new stuff you can't screw into both ends, you'd need a union that would require threading both ends of the pipe, one with right-hand threads the other with left-hand threads, so when you tighten it up, both sides come together.
I'd think a good pipe dope would be better for you instead of teflon tape.
Unscrewing old pipe might be a major pain, things could break. It could be fairly easy, or it could be a major pain and you'd end up replacing more pipe than you want or plan to. Any disturbed joint could break or leak.
See what the pros have to say...