I'm trying to make it so that when an effect is cleared from the player, a different effect is given to them instead (eg: if speed is cleared they get slowness instead). This works fine when using the /effect clear minecraft:speed command, but if I use the basic /effect clear command, it throws an error, saying that there is a concurrent modification exception. I assume this has to do with the fact that /effect clear is wiping the entire effect instance rather than just a single effect, and it doesn't like the fact that I am trying to add a new effect while it is still trying to clear the old effects. Does anyone know a way that I could get around this problem?

The issue happens because you're modifying the player's effects list while it's still being cleared. When you run /effect clear, Minecraft is iterating over and removing all active effects. If you try to add a new one during that process (inside onEffectRemoved), it throws a ConcurrentModificationException. Instead of applying the new effect immediately inside onEffectRemoved, store it in a temporary list, and then apply it safely during the next tick (in a tick event). That way, the original clearing process finishes before you add anything new.

that is 1. In onEffectRemoved: just remember which effect to apply next. 2. In tick: apply the effect from the stored list.

More likely this prevents any conflict and works even when all effects are cleared at once 

I hope it helped.