Speaking as one who is currently in the process of running Return to the Tomb of Horrors, I would definitely suggest going the full route.
MILD SPOILERS
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The adventure itself is full o' deadly goodness. However, the tomb is less deadly because the players go in forewarned by a journal they find.
As for conversion, most of it is fairly easy to do. Check out my story hour (link is in the sig

) for some ideas. In general, conversion involves replacing 2e stats with 3e, but for the most part, the creatures used are already covered in the MM, CC!, CC2, or Creature Collection. Some of the ones that aren't will probably have some incarnation in MM2.
The Tomb of Horrors has already been converted on this very site. However, you may want to peruse the thing first, because the person who did the conversion really lessened the deadliness of it. For instance, he replaced the save or die poison with 2d6 con poison or something like that. I ramped that up to 4d6 Con from Congealed Negative Planar Essence, and ruled that it isn't a poison per se, and thus is unaffected by many spells that would affect poisons. I also added an unhallow to the area, and tied a dispel magic to it to slowly strip away the buff spells a party constantly uses. The demi-lich I expect to eat one soul, or two if it gets a good initiative, mainly because the party has a Paladin and two fighters with +5 swords (from Greater Magic Weapon), and the demi-lich doesn't have a heck of a lot of hit points (only 52 on average).
Overall though, it has been a blast so far. I'll be following it up with the Bloodstone series. The party will become the lords of the Bloodstone area if they can stop the spread of undead in the area. That mens they'll get all the bonuses and hassles which come with the property, including Orcus himself.
