So it makes sense to use single spaces exclusively. Here's an easy way to convert, but there's a slight danger. The key is to use Search and Replace intelligently. If you have used spaces for tabs, make that switch first. You can always switch back, but as you'll see, you probably don't want to do that.
Tabs are almost always better.
If your habit is to press the space bar four times, then replace groups of four spaces with a tab. If, on the other hand, you use three or five spaces, then replace groups of that number. Below, I'll presume you use four spaces for tabs. First, change any string of four spaces to a tab character. Now click Find.