GP: What difficulties arise when converting an old D&D module to a new PC game and how have the difficulties been combated?

We had to convert a lot of monsters and NPC’s to the new rules, as well as their magic items and other loot. Some 1st edition magic items and spells don’t exist in 3rd edition and conversely, there are a lot of new features as well, especially feats and skills, to give to all of the NPC’s. Currently, we are encountering some problems balancing the monsters in the module against the player character. The 3rd edition characters seem much more powerful than their 1st edition counterparts. But luckily 3rd edition rules cover how to increase the challenge rating of monsters, so we can improve the monsters and still stay in the scope of the rules.