Characters for Khmer Vowels

[kiragecko]

I mostly focus on the Angkorian period, and I'm confused with how Angkorian vowels line up with modern ones. When the Middle Khmer sound shift happened, 'u' and 'o' seem to have been conflated into one symbol. Did the original 'u' symbol just disappear? I can't find any modern vowels that look similar.
It just seems weird to drop a vowel and then make a bunch of new ones, instead of reusing the old vowel for something else.

[panini]

If nobody here knows, you could try the linguistics stackexchange, where I think some occasional users know the history of Khmer.