I agree with Saxon about replacing the ring joint every time the flanges are separated; however, many folks incorrectly reuse the ring joint. Provided that the stud bolts have not been over tightened they can be used again. If stud bolts are stretched, they should be discarded. The problem is being able to determine if the bolts were over tightened in the past.
Many years ago, I worked for a major oil field service company that would reuse the rings and hammer the stud nuts tight. The stud bolts were never torqued, and as such, I have no idea if we were over tightening them. The same company did not change the packing on their coil tubing equipment as scheduled by contract. They saved $$ by not doing the job correctly. Fortunately, I had the good sense to quite before their reckless practices created an oil spill or injury while I was in the field.
Because you don’t know who tightened the stud bolts in the past, it may be good insurance to replace the stud bolts and nuts every time as well.