The A course stays at .015 in both. Medium raises the E, D and G courses, giving those courses a firmer feel. The G and D courses are wound with 92/8 phosphor bronze over a high carbon steel hex core, and the A and E courses are plain steel.
You can mix gauges if you understand how the change affects each course. We make phosphor bronze wound loop-end singles in .024, .026, .036 and .040, plus plain steel loop-end singles. Keep the two strings in each course matched.
Singles also let you replace one broken string. If the other string in that course is badly worn, replacing both keeps the pair consistent. After a gauge change, check how the mandolin plays. You may notice changes in feel, action, response or intonation.