Is it perfectly acceptable to take off a fitting and solder a coupling to it, as opposed to going back and soldering to virgin pipe?
Nothing wrong, except that it is more difficult to re-use a fitting or resolder to a length of pipe.
The solder residue from the previous time might be too bulky to fit the parts together.
For one reason or another, the joint made with re-used parts often develops oxidation inside during reheating and the solder doesn't seal the joint properly and it leaks.
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