[zamadatix]

Ah "nextafter" is indeed what I was looking for it just isn't in the JS standard library or Python version I use. Google has plenty examples of the function once you know what it's called though.

Complexity wise that actually seems to give an equally simple "shortest answer" method - nextafter up and down and using text processing find the first digit that changes, see if it can be zero, if not choose the lowest value it can be an increment by one, remove the rest of the string accordingly, and right trim any 0s from the resulting.