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?"
In this context, it is more like: "I know how to drive. I have been driving for a long time. But I have never driven a massive pickup truck (or super fast sports car, or whatever) before. Any tips before I get behind the wheel?"
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.
None of this sounds remotely accurate to me, having played characters of both levels. There's nothing equivalent of Wall of Force or Teleportation Circle available to 5th level PCs and many class features unlock at those levels that change the way the game plays. Adding hit points and to hit bonuses and increased damage to lower level enemies doesn't really adjust for the PC capabilities, either.
But, hey, you do you. I was asking for advice. If you don't have any for me that's fine. I just don't know why you felt the need to tell me I was wrong for asking for it.