Well,saying that the only thing you need to do is "approximately invert the ACs" is not correct IME.
"prettymuch the only thing," the only thing I found the need to do, consistently, with every critter. Specific stats/saves/skills/etc often don't come up, for instance... and, if they did in 1e, I'd be looking them up or making them up, anyway, so I don't really consider that a conversion of the module.
1. Monsters- the ACs have to be changed. Monsters don't have stat blocks (or statistics), so you have to approximate what their stats would be for everything from "to hit" bonuses to saving throws.
1e stats can be very brief, AC, hps, d/a, &c, yes. AC's the one I see a consistent need to change. Filling in blanks like attack bonuses you can do, or not, depending on how consistent and 'fair' you're intent on being.
2. Effects. Again, there are a number of save or die mechanics (must modify).
If a monster's poisonous, you can use the poisoned condition, if you like.
3. Treasure. I assume you're not handing out 1e treasure, right? Right?
I barely hand out treasure, at all. ;P When I run a 1e (or Basic, I run In Search of the Unknown now and then - different every time, of course) module, it's usually a one-off adventure, the players won't get experience and play the PCs again, they're typically pre-gens that go back in my folder. If there is treasure that's a highlight of the module or a McGuffin item, sure they'll find it, but it has no bearing outside the context of the module.
Now, conversions can be easy (either on the fly, or by subbing in 5e monsters for the 1e equivalent).
Where short stat blocks aren't provided in an old module, just pulling up the 5e stats is usually fine, at lower levels, anyway, they're not often far off in terms of CR.