In most cases, your belt tension won't need to be adjusted to accommodate a 13" rear shock. There is an acceptable variance in belt tension, meaning the swingarm can move up and down a given amount without negatively affecting belt tension. When you go from a 12" shock to a 13" shock you will still be within this acceptable variance of belt tension. With our 13" shocks, we recommend 1" of sag, so in actuality, the shocks will sit at a 12" bolt to bolt length when the rider or rider/passenger weight is on the bike.
With that being said, if you are unsure if your belt tension is correct, we recommend referencing your Harley Service manual to verify the belt tension is set correctly.
As a side note, if you were to go with a 14" shock, the belt will more than likely need to be re-adjusted to account for swingarm movement outside of the previous range of movement the belt was adjusted for.