I worry a little about the conversion of ToH that we will see in Tales of the Yawning Portal. Modern players are too used to getting things their own way - instant and irretrievable death is quite hard to come by after the 1st few levels.
Tomb of Horrors has a reputation to keep. It is meant to strike fear in the hearts of any prospective player. It was THE hardest 1E adventure, even tougher than the original Ravenloft, and that is saying something. It was designed to grind cocky players into the dirt, and should never, ever, ever have been dropped into a campaign where the players were using long term characters.
It was quite simply THE best module for a one shot game though. Bar none. Anyone who argues differently is plain wrong.
So if there is any watering down of the difficulty for 5E then it will cease to be the Tomb of Horrors. The previous 5E conversion was abysmal for this reason. There must be sections which cause instant death (no save), there must be multiple instances of save or die. There must be explosions which do twice the average hit point damage of an on-level Wizard. There must be puzzles which requires serious inventiveness to solve.
And Acererak needs to be a lot tougher than he is the MM.
I DMed it twice for my 2 groups using the original 1E rules. I created a specific party for the adventure including a 13th level Cleric, a 14th level Mage, a 12th level Monk, a 12th level Paladin, and a 7/7/9 1E Bard. They had top drawer magic items - A Staff of Power, a +4 Defender Sword, Wings of Flying, Slippers of Spiderclimb, and many more - their spellbooks and initial spell selections gave them massive hints as to what spells they would need - Commune, Forget, Shatter, Find Traps, Teleport, Locate Object. I created a group of spare characters who would wait outside the Tomb to replace the inevitable casualties.
Acererak never even got scratched on either occasion. One group lost all 5 of their starting characters, didn't use any of the spares. The other group used some of the spares, one original character (the Mage) survived - he fled the Tomb naked having seen all his companions blown up, squashed, dissolved, impaled, and entombed.
11 dead PCs, 1 traumatised survivor, the Demi Lich continued to sleep soundly - untouched. And that's the way it should remain.