In Hammer, when I change a func_detail to func_brush, it always crashes.
However, if I somehow get a func_brush in there first, it only crashes occasionally.

Has anyone else encountered this problem or know how to fix it?

I'd suggest you that whenever you have to change a group of brushes from any entity to a different one (like in your case), first move those brushes to world (Hit Ctrl+W), and then tie them again to the desired entity (with Ctrl+T). I'm not sure if this is your case but, normally when you do as you described, it suddenly appears a new line into your entity's properties saying "spawnflags" and its value is "2" (I think ). This is consider by the engine as an error, and indeed you'll find it when you go to hammer Map > Check for problems (appears as "unused spawnflags value").