The Powder Toy has a neat little history function built into the search box. To access it you need to:
1. Find your save ID. This is located at the bottom of the box that pops up when you preview a save. Here's an example on one of my saves:
2. Type "history:" into the search box then paste your ID after it. This will bring up all the iterations of that save that have been uploaded to the servers. Example:
3. Click on any of those and open them as you would a normal save. You can then resave it as the main save if you wish.