UITableView has a neat trick to animate changes in cell heights: if you send an empty updates block to the table view, it won't insert or delete any cells, but it will still recalculate cell heights for the entire table. So even though this piece of code looks weird, it should do it:
[tableView beginUpdates];
[tableView endUpdates];
If you want to change the contents of the updated cells, too, you must add one or more calls to reloadRowsAtIndexPaths:withRowAnimation: to the updates block, of course.
