I didn't ask about 2E or 3E. I asked about 5E because that's the edition that I dont have much experience running in the 9th level range. So since I know "how to run D&D" in a general sense, I was asking for specific advice as it relates to 5E at that specific level. I'm not sure where your confusion comes in since a number of folks have been able to provide specific advice, but maybe this clears it up?
Not really. 30 years of experience - exptensive 5th edition experience... and apparently vastly expansive experience running 5th level - but you are unsure of what to expect from 9th level in 5E - but don't want general advice?
It is confusing. It is a bit like saying, "I've been driving a car for 30 years. I've driven a number of different cars. I don't need people to tell me how to drive a car. However, can someone explain how you back up a car?"
The above advice that has been provided is pretty general. Listing HP, AC and Passive Perceptions is not level specific advice.
There is some stuff specific to certain spells (animate object), but if you're run a lot of 5th level adventures, these types of spells should come up in the hands of enemies yo've run, right? And your individual rule tweaks (Do you allow flanking to grant advantage, for example? Does it require a reaction and check to identify a spell being cast?)
9th level versus 5th level is mostly similar... things really change the most at the tier marks (5, 11, 17). Yes, you get some new character abilities, some new spells, but they're all still formulaic with existing spells you've seen at 5th level. The differences between a 5th and 9th level PC isn't that much different than comparing two different 5th levels PCs.
If I'd run dozens of 5th level adventures at cons and was now planning to run a 9th level adventure for a different group of players, I'd just reuse the 5th level adventures and bump up the monsters.