Converting AD&D critters to C&C can be done on the fly : Hit Dice, Hit Points, Attacks, and special abilities can be taken as they are. You just have to change AC (simple math: 20 - AD&D AC = C&C (or d20) AC) and use your judgement and C&C rules to adapt the saving throws.
Anything else can be adapted on the fly, such as the difficulty of traps, where you just have to figure the approximate level of the guy who's done it, poisons are less deadly but there are rules in the C&C PHB to handle them, spells can mostly be used as is, etc.
It's much easier than a conversion from AD&D to d20, where some monsters received such a notable boost (giants, dragons) that they cannot be kept as they were if you want to play with the previous AD&D players levels, and where you feel you have to add some d20 "sauce", like skills, feats, prestige classes, advanced monsters, etc. to the mix.
I am actually running a HARP Middle Earth campaign, a D&D 3.5 AP3 - Paizo - Savage Tide campaign and a C&C Queen of the Spiders (G1-3, D1-3, Q1) campaign, and I have to say that each of these games has a different flavor:
- HARP is sweet and delicious, full of little details I love DMing, dark and bloody on some occasions. It conveys the setting's atmosphere so well that this campaign has become a role playing paragon for us (some of us have played RPGs for more than 25 years). I think HARP will stay as our ME system for years, as RM or MERP have been for others.
- D&D 3.5 is a techno fiesta I don't enjoy that much as a DM, but my players for this campaign are like the "dream team table" of our large gaming group, so I share some very good moments with them. I won't start another game of D&D 3+ after this one.
- C&C is like DMing in a rocking chair, with sun glasses and a good drink at hand, as a DM you just have to make it fun, quick and let the monsters and the epic adventure do their job. Never have a look at a rule book, just play.