Let's take a look at the final battle in The Sunless Citadel. Per the module, the characters should be 3rd level when they have this fight.
The NPCs and monsters - Belak, Sir Braford, Sharwyn, the giant frog, and three twig blights - add up to 775 XP.
Under the 5e model, the number of monsters multiplies that to 1900 xp, or a deadly encounter for a 3rd level party of 4 characters.
Under 5.5, 775 XP is a moderate encounter for a 3rd level party of 4. To push it up to high difficulty, we technically need to add 130 XP to exceed the moderate listing, but that doesn't feel right.
I'd expect to aim for 1600 XP for a high difficulty fight, giving us 825 XP to add. We could clone each of the creatures and still be short.
In other words, to make the final fight in a 5e adventure work you need to double the threat or add a CR 3 and a CR 1/2 creature to the fray.
EDIT TO ADD: That also means that Belak is no longer the toughest NPC or creature in the encounter. He's CR 2.
That's wild. I'd expect them to keep the XP and CR values the same, but then make the individual creatures more powerful.