Tuning a flat reed

If a reed drifted flat by just a little it can just be metal fatigue and I would tune it up. I have done this before with good results that lasted for years. However, if the reed went very flat and/or this happened very quickly then assume a crack and replace the reed. Also if you tune up a reed and find it is flat again a few days later then assume a crack and replace the reed or reed plate. Although rare, a cracked reed can break off and you can actually inhale it. A trip to the ER is more expensive than a new harp. Plinking might help you find if the reed is cracked. I always plink after tuning anyway to help the reed settle. Most of the tuning I have done has been to make tiny tweaks to new harps or to retune to a different configuration (making a Melody Maker or Paddy Richter out of a standard harp). I can think of only once or twice that I needed to touch up a reed that had drifted flat. I have never had a cracked reed or a broken reed. I think that happens more to blues players who try to wail over an electric guitar.
My diatonics are all Lee Oskar. They sell replacement reed plates at a reasonable price. I think some of the other brands can sell new plates also. I would not bother replacing a reed on a diatonic. I have bought a couple pair of reed plates for my 20 diatonics over the past 15 years. Those reed plates could probably have been tuned up but there were multiple notes and the plates were years old so I just went with new. Oskars are tough. For a tremolo I would go to the trouble of replacing a reed, although I have never had to. Tremolo harps cost more and I have time invested in several of them fine tuning the tremolo rates.