So, I've been playing through Dragon's Curse, and I got to Piranha Man.
Now, I managed to miss the sword that destroys blocks, and so I can't change back to Mouse man in the town. However, I need that sword to progress further in the game.
Do I have to restart?
I don't think there is ever a need to restart...you should be able to get back to any area where you might have missed an item that will help you get to the next area (but I could be wrong since I haven't played it in a while).
If you go back to where the pyramid boss was, i think you can turn into the mouse