12pm is now always “noon” and 12am is now always “midnight”. You may say things such as “noon thirty” to indicate “12:30pm” or “midnight forty five” to indicate “12:45am”. Similarly, talk about “noon plus 3:25” in order to indicate “3:25pm”.
So the problem is that anytime you have some sort of modulo arithmetic people fail spectacularly. I think it’s because discontinuities is something that can generate Cognitive complexity  and modulo arithmetic has a giant discontinuity built right into it.
Technically the new Vocab Guideline still has a discontinuity in it, but it’s a discontinuity that is a bit easier on the mind. AM is from the latin ante meridiem (before midday) and PM is from the latin post meridiem (after midday) . This adds two difficult things for the individual to process in addition to the notoriously bad modulo arithmetic. Namely an acronym from a dead language, and the disconnect between solar noon and 12pm and midday.
So making people think about a latin acronym is bad because most people don’t know latin. Now they have to deal with something completely out of left field when they’re already having problems. The other side is that midday used to actually be a meaningful concept before our modern conveniences. You got up at sunrise because that’s when you could see and you went to bed at sunset because that’s when you couldn’t see. Midday was solar noon because that’s when the sun was in the middle of the sky and you knew the day was half over. Today solar noon and actual noon don’t match up, and it doesn’t matter anyway. We have alarm clocks, meetings with the other side of the world, internet, lights, 24/7 stores, and uber. Noon and solar noon have nothing to do with whether or not the day is half over.
This actually isn’t the first instance of something that doesn’t make a lot of sense that we still use due to tradition, but totally made sense back in the day. You might think that a mile as a unit of measurement doesn’t make a lot of sense at 5,280 feet. Shouldn’t we use the metric system or something. Well we inherited miles from the Roman mile, which was defined as 1,000 paces . And that sort of sounds like the same thing that we get from the metric system. It’s almost like they did something that made sense instead of having a totally arbitrary system.