Once it's done, it's done. You can prevent such mistakes in the future by going to Tools>Options>Charts and checking the Use 'Alt' key to drag trade levels.
That has got me a number of times, so I turned off "One Click Trading" under Options -> Trade.
It means I get the popup window, then I have to hit the Modify button to continue. It's extra steps, but it's fine if you are a swing trader.
Why not just turn on the 'Alt' key to drag trade levels? Then you can mess around with charting all you want and it won't interfere with your orders but you can still drag orders if you need to.
That doesn't solve the problem of intentionally moving the SL (e.g. to visualize in another place), then deciding that you didn't want to.