This is a question of aspect: specifically, the difference between “perfective” and “imperfective” aspect.
(Be careful: perfective has nothing to do with the perfect construction using HAVE + past participle.)
A perfective sentence speaks about a completed action, one which has reached its end, one which is ‘over and done with’. The simple past form of a verb ordinarily reflects a perfective meaning.
Jack ran to the corner —we understand that Jack ran until he reached his goal, the corner, and then stopped running.
(Note, however, that simple past and simple present are not symmetrical. The simple present is not usually perfective: in ordinary circumstances we don’t use it to speak about completed events, because an event which is completed lies almost by definition in the past! —But there are exceptions, such as sports broadcasts where the action unfolds ‘live’: He shoots, he scores!)
An imperfective sentence speaks about an incomplete action: within the sentence itself we see only the middle part of the action, after it has begun and before it has come to an end. The progressive construction with a verb ordinarily reflects an imperfective meaning.
Jack was running to the corner —we understand that Jack was in motion toward the corner, but we don't know at this point whether he actually reached the corner. We may or may not find out in a later sentence.
Now let’s take a look at your example:
Ben ____ to go to his high school class reunion tomorrow, but now he is not sure if he can.
In this sort of context plan is almost certainly used to mean “intend” or “expect”, and that sort of meaning is inherently imperfective: you keep on intending to do something until you actually do it (or actually fail to do it, or decide that you won’t do it after all). In this case, in fact, we are told that Ben’s “plan” is now up in the air: presumably he still wants to go to the reunion, but he doesn’t know whether or not he will actually be able to.
So there’s never been a definite end to Ben’s plan—it’s not in any sense complete—and we use the progressive past: progressive to indicate that the plan is incomplete, and past to indicate that the plan was in the past pretty definite, even if it’s not so firm now.
If you used the simple past in this present context you would be “recategorizing” the verb plan as something with a “perfectible” meaning: a meaning which implies a definite completion. And in fact plan has a meaning like that: “make arrangements”. If you said Ben planned to go tomorrow, but now . . ., it would probably be taken to mean that Ben made arrangements to go, but now something has happened to upset his arrangements.