Simplex AD&D
Character generation
Low-level characters can be transferred into Simplex with relative ease. For a first-level character, start with the usual 100 experience points; for each next level, add the current level times ten. So a level-three character would have 130 points, and at level four 160. Converting high-level characters is not recommended, nor easy, though, and I recommend that all members of the same party get an equal amount of experience. Work out your character normally; there are no restrictions on taking skills, for instance if you want someone to be able to fight and cast fireballs (i.e. dual class fighter/mage) that is easily possible.

Fighters are the easiest to convert. However they will have to get used to being very vulnerable, because even the toughest guy can be cut down with a couple of well-aimed sword blows. On the upside, they gain a much larger scope of abilities.
Wizards are most difficult to change, firstly because they tend to have high intelligence and therefore a lot of skills, and secondly because the magic system is very different. A low-level wizard will be less versatile, but can cast many more spells per day.
Thieves might have a problem in converting their thief skills. However, most of these would be at rather low percentages. If any score is below 50%, you probably don't need a rank in it, and anyway a high dexterity and/or perception would cover for most thief abilities.
Priests are still restricted by their religious ethos, whichever that is. Other than a high faith score they can pick whatever skills they like and can thus be easily converted.

Ability scoreAttribute..... Score Rank
Strength Strength 4-7 3
Dexterity Dexterity, Agility, Reflex 8-9 4
Constitution Endurance, Anti-magic 10-11 5
Intelligence Intelligence, Knowledge, Perception 12-15 6
Wisdom Willpower, Empathy, Faith 16-17 7
Charisma Charisma, Persuasion 18+ 8

The secondary ability scores from the Skills and Powers book are more easily mapped to Simplex scores. I'll include them once I've looked them up.
Dungeons & Dragons Simplex