A bit of advice from my own experience with the Tomb of Horrors:
The most critical thing you should do to make the Tomb a fun adventure is to place harsh time pressure on the PCs' expedition. If they are able to take the time to search and prod every 5-ft. square, let alone retreat and recuperate several times, they'll "brute-force" much of the adventure and it will just be a boring slog.
The dungeon doesn't have many combat encounters, and I'd advise you not to change that -it would really change the tone of the Tomb, and not for the better, IMO. There are four instances of monsters that you should make sure provide the proper level of danger, but the rest are mostly detritus and serve only to deplete party resources.
Those four important challenges are:
- The Four-Armed Gargoyle near the entrance to the dungeon, which should be a real slobberknocker, such that its presence encourages the PCs to try another route
- The "false Acererak" (the least important of the four, but it should really be a serious fight in order for the misdirection to be convincing)
- The ethereal demons around and throughout the Tomb which discourage ethereal exploration/bypassing the dungeon (A brilliant idea which I'm surprised not to see more often in modules, but the demons need to be appropriately scary for the deterrent to work)
- and of course Acererak himself. (The 2e demilich stats are horribly arbitrary, decide whether you want it to be feasible to fight Acererak or if he's just a horror that the PCs should avoid/weather while they loot the final chamber, then make proper stats accordingly.)
Some of the traps in the Tomb you will probably find to be goofy or inappropriate. I won't call any out, since it's all in the eye of the beholder, but be aware that you might have to modify or replace some material beyond simply deepening the pits and increasing the save DC on the poisons.
Remember, when you're scaling up the tomb, not to overdo it, since the time pressure will force the characters to continue against their better judgement, and even traps which aren't lethal dangers in themselves will wear down the group for what lies ahead.
If you want more opportunities for roleplaying, take a look at Return to the Tomb of Horrors. I wouldn't run "Return" itself in its entirety, but the idea of Skull City, that the tomb of Acererak became a site of pilgrimage for various undead and necromancers, is a good one. That provides opportunities for the PCs to negotiate their way and learn information about the Tomb before they go in, or face dangerous and intelligent enemies if they're not careful. If they have to sneak their way into the Tomb, leaving might not be safe, and if they fight their way in they might have vengeful necromancers on their heels or trying to sabotage their expedition.
Edit: The conversion made by Justin Alexander, of the Alexandrian, is an excellent starting point.
http://www.thealexandrian.net/archive/archive2005-11.html#20051115
