Ideally, to widen nut slots, it's best to have real nut files. However, most players aren't gonna run out and buy $90 worth of files just to work on 1 nut.
I've found that using a gray piece of fine grain sandpaper and creasing it in half works for D-E strings, and on the heavy strings........use automotive feeler gauges or a piece of guitar string with the gray sandpaper folded around it's edge. If' you're fairly adept at doing precise jobs like this, go for it. If you ever go back to light strings again, you can do it if the slots were only widened for 49-50. If you're using really fat strings, you'll need to install a new nut, so the new strings aren't bouncing around in the wide slots. Whatever you do, don't forget to straighten your neck using the truss rod and re-intonate the guitar after you change ANYTHING regarding strings. And I wouldn't recommend stepping up to fat strings if your guitar his a skinny neck like an Ibanez Wizard neck. Schecter necks or fatter can handle it.