Some people, even much admired members of this august community, have claimed that an adventure is an adventure is an adventure, that you could use a D&D supplement in an Ars Magica game with only a small effort.
I beg to differ. I see several pitfalls along the way.
To place it all in context, I'll be talking about how I think a particular supplement can be converted to Ars Magica - The Shackled City Adventure Path. The SCAP is a full fledged D&D campaign, stretching levels 1 to 20. It begins [SPOILERS!] with the PCs finding and releasing kidnapped kids from an underground hobgoblin slave trader. It is set at a city built on top of a volcano
- The Implications of Combat
Combat in D&D is a fairly brisk thing, and recovery too is fast and furious. D&D adventures often are, or include, "dungeoncrawls" - hacking your way through a dungeon.
The PCs in my game spent days hacking their way through the gnomish enclave below the city. They slept in the dungeon, recovering their wounds and being fresh in the morning. All this combat would have left ArM PCs terribly wounded, unable to proceed for months. And very likely dead.
I think the amount of combat definitely needs to be lessened. This actually is the least of the concerns. The plotline wouldn't have been seriously hampered if the resistance was lesser.
- The World Assumptions
The world D&D adventures assume is different from Mythic Europe in many ways. Many times adventures depend on such world elements, and including them requires twisting or abandoning Mythic Europe completely.
The hobgoblins kidnapped the children to sell them as slaves in the underdark reached through caves below the city, using nearly-invisible critters that stalked the streets at night. The opposition included a half-fiend dwarf with a demonic hound, hobgoblin mercaneries, a strange underground "client", a guest appearance of a beholder, "skulks" and "dark creepers" which are stealthy little humanoids living underground. Further underdark denziens (powerful races such as drow - dark elves) were intimated. Further adventures will involve alternate planes of existence (hell, for one), the eruption of the volcano the city sits in, independent wizards (many involved heavily in local politics), retired adventurers, powerful clerics of many faiths (several of whom live in the city), a vast area under demonic control (Infernal aura), and more...
Generally speaking, I think it's best to downplay the D&D elements. Take the plot, but put it in the ArM context. I think it creates a far better game.
For the SCAP, I would do the following: have the city not be on top of a known volcano, but rather near a long-dead one (plenty of those on earth). The enclave below the city would be a faerie-land, an area of Faerie aura once populated by gnomes/dwarfs [as in the original D&D version] but now overrun but darker faeries. The kidnapped children would be "sold" to even darker beings in Arcadia, as part of a deal by the place's ruler [the "half-fiend" dwarf] to satisfy the faeries' desire for human interaction in deep dark reaches of Arcadia.
Further plots will likewise be modified. For example, at one point a cleric of the good deity is assasinated when traveling from a city carrying wands of control water to be used in controlling flooding. It's just not reasonable that a cleric in ArM will go to purchase (magic!) wands in the big city. Some other ploy might be used instead, however. Perhaps he was returning with a relic when he and his entrouge were attacked. [By were-baboons... lot's of conversion needed!]
The most difficult part of this is are the basic assumptions. The Order - there simply isn't one in D&D. Nor is there a unified Church, nor is the party (or its leaders) largely shun by the populace (the Gift). Nor are there adventurers in ArM. This is very difficult to accomodate - just how can the magi involve themselves in the politics of the city, as the SCAP assumes they do? How can they develop the positive reputations they are assumed to have? How will the Order react to it? How is the Church related to the various polytheistic "churches" in the city?
- Rate of Advancement
Another big issue is the rate of advancement. In D&D, the rate is very very fast in game-time. The entire SCAP takes places within a few months at most, and yet the characters begin by facing minor threats (level 1) but end up facing powerful demon lords and archwizards! This rate is just untennable for an Ars Magica game.
There are only two ways to resolve the issue. One is to abandon ArM's slow advancaement and focus on adventure XP and allowing quick-learning of spells. This I think does great disservice to the setting and game.
The other option is to slow down the rate of the plot advancement. This is sometimes problematic. Again, the entire SCAP is supposed to take place within a few months and some elements of the plot can't reasonably be delayed. For the most-part, I think the SCAP in particular can be slowed down, at least the initial stages thereof. (The last few stages are better connected and cannot be unduly seperated.) Even so, the plot could run only across a few years at most.
This leads to a problem in adopting D&D adventures that span many levels. Adventures that span only a few levels are much better in this respect. An adventure that spans 20 levels, like SCAP, is almost impossible to convert.
Specifically, for the SCAP, I would significantly lower the power-level of most of the main-players, making them not very powerful actually. This won't work for some of the NPCs, that can't possibly be weak while preserving the plot. I'll either skip or cricumvent their defeat or introduce a mcguffin to let the PCs prevail (a "true name" and Hermetic Synthemeta to let the PCs ignore Might, for example).