I was scared too to break everything but one really out-of-tune note forced me to do so. I ended retuning the whole keys. I expected the reed to be much more thin, so there’s a good margin until you break something (Hohner26 for me).
The reed gapping adjustment was a more difficult process as i learned the gapping didn’t have to be uniform (the high-pitch keys gap have to be REALLY REALLY small… i had some difficulties with the last 4 keys).
I also learn that for playing my style (reggae roots), a well tuned melodica doesn’t sound too good for that .. but that’s another story. From now i would probably just only tune some specific keys which don’t sound good.
I would really recommend the tuning experience, you will gain much knowledge on your instrument.