There is an item in the Magic Item Compendium (belt of battle perhaps?) that uses charges (as a swift action) to give yourself extra actions in a round. I believe one charge gives you a move equivalent action, two charges give you a standard action, and three charges give you a full-round action.
So, for example, in one round you could:
1. Move into position (move action)
2. Cast a spell with a casting time of one standard action (standard action)
3. Spend two charges on your belt to give yourself another standard action (swift action)
4. Cast another spell with a casting time of one standard action (standard action)
But one of my characters wants to cast a spell requiring a swift action at step four, not a spell requiring a standard action. His argument is that a standard action takes longer than a swift action, so the belt should allow him to take a swift action at step four rather than a standard action.
However, the item doesn't explicitly allow a user to grant himself a swift action--only a move, standard, or full-round action. I argued that swift actions are not interchangeable with standard actions, and thus I ruled against him originally.
So, given all of the above as an example, does the RAW support the player's being allowed to use a swift action at step four, and if not, would you still allow it, and why?