I believe that the ritual solution is the wrong solution. It is a Have vs. Have Not solution, and the game already has too many of those. Having thought on this for 24 hours, I would recommend making a change that allows players to "slide" build after every PC'd game.
The inspiration for this idea is the current rule that allows a player to completely respend their build after their first game ever. Accordingly, my suggestion is this:
After a weekend, a player can remove up to 15 build worth of skills, turning it back into unspent build. New skills can then be bought (following all applicable teaching rules) with that build and with any other free build (including build gained that weekend).
[The actual value is 7.5 build a day, so that faire days and long weekends can also be accommodated]
How this plays out: Early in a character's career, a character can completely switch build expenditures quickly (2-3 events). But this probably won't come up much since most PCs don't start considering a Spirit Forge until roughly level 10. At level 10, it would take 7 events (slightly more than a year for the average chapter) to fully transition to a new skill set. In the mean time, the character would be somewhat hampered by a transitioning build (though the fluid class system that is already in place would help with this).
Also, because it is a transition, the change would seem more natural in-game. Imagine a level 10 fighter transitioning to caster. The fighter starts swinging 8s. After the first weekend, the fighter swings 7s and learns to cast a single 1st level spell (and can now read and write). After the second weekend, the fighter swings 6s and can cast a small pyramid. After the second weekend, the fighter swings 5s and cast cast a slightly bigger pyramid. Around weekend four (it might have happened last weekend, I didn't do the math), fluid classing kicks in and the fighter is now a templar with roughly even sword skills and castings skills. Over the next three weekends, the character loses all weapon skills and ends up with a 4 column and a little high magic.
It is very organic and not too abrupt (except at low levels where it isn't a big deal that it is abrupt). Also, keep in mind that the character is still gaining build after every event, so the character is actually gaining about 18 or so build worth of casting skills each event rather than 15.
Just to be clear, I would still keep Spirit Forge in the game, allowing players to spend massive amounts of resources to make the change instantly rather than at a slow burn.
-MS