Wizards are amazingly simple to convert. You won't get precisely the same thing, since RuneQuest sorcerery is more flexible and has fewer base effects. First, abandon the Free INT rules for RuneQuest Sorcery. This is the one absolutely firm decision of the folks designing the next edition of the game. Free INT limits on manipulating spells is out the door. A replacement for this limit would be one-tenth the sorcerer's Intensity skill.
Multiply a wizard's AD&D level by three and add this to 50 to get the sorcerer's Intensity skill. Multiply the wizard's AD&D level by ten and divide this among the three sorcerous manipulations.
A sorcerer would know 2 sorcery spells per old AD&D level, and they would have 20 percentiles per level to divide among their spells. Get to know all the sorcery spells before you turn your players loose upon them. Some of them need special attention and might not be appropriate. Furthermore, there are a lot of "spells" that are actually dozens of spells with similar names.
For example, the Summon (Creature) spell does not permit a sorcerer to summy any creature. Summon (Creature) is a shorthand to indicate that there is an infinite variety of "Summon" spells, and each spell is for a specific type of creature. Thus, Summon Ghost, Summon Nymph, and Summon Salamander are all specific spells that fit within the category of Summon (Creature). All three of these spells have to be learned separately. A character cannot simply learn Summon (Creature) as a single spell. Note that a "Summon Animal" spell would be fairly useless, since it would only summon the closest animal, even if that were an ordinary ant. You cannot specify a specific instance of a creature unless you know an individual's True Name.
You might want to assign PC spells instead of letting your player pick them to preserve more-or-less the same flavor of magic their character already knows.
RuneQuest has no analogue to the spell categories of AD&D, so simply convert the spell abilities of Specialist Wizards.
Priest probably cast, oddly enough, Divine Magic. They also are likely to have access to Spirit Magic to round out their abilities. You have to do some serious work for your world's religions (if you have not already done so). You need to decide what spells each deity or religion teaches its priests. Unlike AD&D, there is no such thing as a generic "cleric". All priests are reflections of individual deities, and their magical choices are, thus, limited.
Fortunately, RuneQuest's chapter on Divine Magic has some fairly useful guidelines to help you decide what magic is appropriate for various types of deities. For example, a generic War God might teach the Divine spells Berserk, Shield, and True Sword. He might also teach the Spirit spells Bladesharp, Demoralize, and Protection. You can also mix different types of deities. A pantheon might have a god who combines the functions of Ruling Deity and Sun God, like the Egyptian religion. In fact, a religion like the Egyptian would have each deity combining some aspect of the RuneQuest Sun God template with whatever they were. These templates are just templates, not hard and fast molds. Use them as guidelines to help you, not pre-set deities.
Once you have translated your religions into RuneQuest, then you can translate the priests of those religions. RuneQuest has much higher qualifications for priesthood than does AD&D. There is no such thing as a "beginning" priest. If you're a priest in RuneQuest, you've already had to amass a great deal of experience or training. Fortunately, there are lesser levels of devoutness that religions support.
Priests of fourth or lower level could be considered Initiates, albeit favored Initiates. They gain all the standard abilities of Initiates described in the "Divine Magic" chaper of RuneQuest. These new Initiates would know one point of their religion's Spirit Magic per AD&D level. In addition, they would know one point of cult Divine Magic plus one additional point for every two AD&D levels. Remember that the Divine Magic of Initiates is one-use. Once it's used up, it's gone. Many initiates on the track to priesthood thus hoard their Divine Magic.
If you don't like that feature, a rule I've been using is that Initiates can get their cast Divine Magic back, but only on their religion's holiest day of the year.
Priests of fifth or higher level could be either a Priest or an Initiate, depending upon his conduct. If the priest actually knows the skills favored by his deity, raise them to 50% (if not already there) and the character is a priest. If the character does not already know all these skills, he obviously hasn't been serious enough in the study of religion (too much time out adventuring and other frivolous nonsense), and is still and Initiate.
Give the higher-level Initiates magic as for the lower-level conversion. True priests would know the same amount of Spirit Magic as would Initates, but they know one point of cult Divine Magic per level. Furthermore, much of their Divine Magic is reusable, as per the RuneQuest rules.
Paladins and Rangers are a special case, and the RuneQuest concept of the "Lord" can be invoked to handle them. "Lords" are explained in Gods of Glorantha. While a priest is a servant of a god, a Lord is expected to actually incarnate some minor aspect of the god. In general, only militant religions will have much in the way of Lords.
You have to make up or adapt the religions that the Paladins and Rangers of your campaign followed, when it was AD&D, to RuneQuest. Once this is done, decide whether the individual Paladin or Ranger qualifies for "Lord" status. The simplest way to do this is that a Paladin or Ranger of high enough level to cast spells converts to a Lord. If the individual religion has Priests and Lords, then the Lord is a functionary of the cult. If the religion has no Priests, then the Lord also functions as a Priest for the cult.
Lords of either type gain one point of cult spirit magic for every two AD&D levels. Lords who are not Priests gain Divine Magic as though they were Initiates. However, their Divine Magic is reusable. Lords who also function as Priests gain Divine Magic as would a Priest, but they also have to have the headaches and skill limitations of functioning as a Priest.
Bards are nearly as easy to convert as are Wizards. Give them half of the magical stuff a Wizard gets, except for the Intensity skill, where they get two-thirds of what a Wizard gets.