I was wondering if there were any general rules that should be followed when changing existing translation notes. Would it be better for me to reach out to the original translator and ask for permission to change anything? Or is it not such a big deal as long as the quality/consistency of the original translation is preserved?

Depends. Is the original translation a typo-riddled grammatical mess? Change it without thinking twice. Is the original translation functional/literal but lacking style and flavor? Maybe discuss it with the original translator or the discord first.

See the Translation Guidelines. It goes into detail on what you should and should not do when handling translation edits.

There's no real rules for changing someone else's notes, and I rarely do it. I guess my guidelines would be:

• Translation is "good enough" - I wouldn't bother messing with it. You can if you want.
• Translation is really awkward or bad - just fix it
• Translation is completely overlooking a cultural reference or something - maybe leave a comment about it