Guys, it isn't just Delta, and it isn't just more than 5 months in advance that they make changes. I have flights booked on Continental for our vacation in one week

and it was changed 2X.
The first time, I didn't even know about. The second time I found out just by chance we were changed (went to add another person to our party). We were on totally different flight #'s going down and coming back. And the original flight #'s were still in their system. Go figure. Going down, the flight got us there more than four hours later than the original, and coming back got us home two hours sooner.
Anyway, I called reservations and was really nice explaining that all I wanted was to be put back on the original flight #'s that I had purchased (since they were still existing flights). Got a nice woman who wanted to help me out, but said that the flight class or price point I had purchased was no longer available on those flights.

So, I was persistent, and she felt bad once she understood that not only had we been removed from our departing flight, but had been removed from our return flight as well. Finally she said she would get her supervisor. Got put back on the original flight for departure, which will get us there an hour earlier

(but means we've got to get up an hour earlier - blech). Did not change our return flights back to the original flight # because that had us getting to our home airport at midnight, so I lived with departing two hours earlier.
Anyway, apparently the reason our flights were so fouled up is because we were changed in the system once before, but weren't notified. So, when those flights were changed, they didn't keep our original itinerary in mind when they re-booked us the second time involuntarily. I only found this out when I had to call back the next morning because although our flights were fixed in their system, the new e-ticket that they e-mailed still reflected the old flights that we didn't want. The next reservations agent fixed that error, and then said, oh I see what happened, you were switched around once before, as well.
So, the whole point of the story is that this can happen on any airline, not just Delta, and can happen closer to departure, as I went through this not even a month ago, and we're leaving on vacation a week from now.