GUEST,plonkers, you don't say if you're an acoustic or electric player. A 10 on top sounds about right for electric, but a 10 top for acoustic would mean you're either getting a thin sound at the moment or you have a shorter than standard scale length.

Overall, your instrument needs to sound balanced, so if you're using a standard set of strings and are in standard tuning, I can't see why you would take the huge leap from 10 to 13 on top. If you're used to super-light 10s, I would guess the 13 would make the top string pretty much unplayable for you, as well as putting the neck out over time. If you have a particular reason for changing the gauge of one string, do as Richard suggested and go up, I'd say, a thou at a time. You may be surprised by the difference a single thou can make to your playing and to the sound of the guitar.

Hope this helps.

Stower