Upgrading SLX brakes, do I also need SLX rotors?
(Proviso - I assume your existing rotors are in good condition - if not the upgrade is a replacement of a worn component which needs doing anyway.)

It will be fine to use existing rotors.  Rotors come in all shapes and sizes and prices, so it can get confusing, but within the same size, the difference is more about balancing cost, weight and durability, the ability to shed mud and the ability to handle heat. Throw into this mix marketing and sales, and most of the information you will find is driven towards getting you to spend money you do not need to spend. 

There would be advantages in upgrading, my guess is they would be very small and probably imperceptible once riding. You gave no indication how much cash you have burning a hole in your pocket, so we cannot really give you a "Is it worth it? Yes/No" answer.