I had already defined a bunch of tasks, and assigned resources 1 and 2. The durations of the tasks were calculated on the resources Max Units availability at the time of assignment. Later I went back and changed the resource Max Units based on a recalculation of the resources availability. I expected project to automatically change the duration of the tasks accordingly, but it didn't. I had to go and manually change the Units field for the assigned resource on the task to get it to change the duration. Is there any way that MS Project will automatically recalculate the durations of tasks when I change the max unit % at the resource level?
No, MS Project will never recalculate the duration of tasks based on the resource max units value.
Most project managers assign the resources for a different value of % unit than the resource's max % unit value. A resource might be working on two tasks in parallel, or in several different projects. A resource may also be assigned to more than its max % unit value on purpose. So changing this automatically would wreak havoc with many project plans.
I'm afraid there is no better way than to change the tasks manually.