If I ever DM a Carcosa campaign again, I think I will simply eliminate the Sorcerer class, allowing any PC who performs the necessary steps to cast rituals. I have DM'd (non-Carcosa) classless games many times, and it is no problem.
Carcosa has only two classes: fighters and sorcerers. Sorcerers can do everything that fighters can do, and just as well. Sorcerers are basically fighters who can also perform sorcerous rituals (with a mechanical price tag of having to acquire more xp to rise in level). Since the sorcerer class already "contains" the fighter class, there would be no advantage to a multi-classed fighter/sorcerer.
I would not allow a fighter to switch to a sorcerer, because I envision the latter spending 10 to 20 years in meditation and study in order to become a 1st-level sorcerer. I do not imagine sorcery to be something like algebra or Spanish that can merely be picked-up with a bit of study. Rather, the mind must be violently wrenched and molded into an unnatural state in order to become a sorcerer. Sorcery isn't merely a study. It is a way of life.
I would not allow a sorcerer to switch to a fighter, either. How could a sorcerer chose to "forget" two decades' worth of de-humanization and occult knowledge of the ways of the Snake-Men? The only advantage a sorcerer could get from switching to fighter is a lower xp cost to attain higher level. I'd simply tell such a player, "Your sorcerer can of course swear-off performing any more sorcerous rituals. He can become (outwardly) indistinguishable from a fighter. But his brain is too twisted by sorcery to be able to advance in fighting skill as swiftly as a fighter. You lack the relative simple-mindedness of a fighter. You know too much."