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I'm painting and I have to take a big shelf off the wall. The shelf is screwed into the studs because of it's size. The wall is drywall.
Will I be able to use the same holes as before and keep the same strength? Or do I need to fill the holes with something?
Assuming that the screws don't feel loose when you remove them due to inadequate length or pilot holes that are too large, you can reuse the holes. I've done just that a thousand times with everything from knickknack shelves to solid wood doors. The point of concern usually isn't the holes, but the screws. If they were barely adequate before, they'll be risky now.
Since you haven't told us anything about the shelf or the screws, I can't be more specific than that.
I will often take a toothpick coated in a polyurethane glue (like "Gorilla Glue") and insert it into a reused hole before redriving a screw (especially cabinet and door hinges), but if the screw was nice and tight coming out, you'll be okay.
I suggest you to put in a new screw little bigger (0,5 or 1mm thicker). Don't know in US units but online calculator exists for this. New screw will have the same strenght.
Just to be complete: In brick walls I suggest to replace the plugs with new ones to restore original strenght (if regoular self-drill screw are used), if standard thread ('bolt' thread) was used the old screw and plug can be kept.
