That depends on the recipe you are using in large part. In most cases you will treat the new batch just as if staring from scratch since re-using brines isn't recommended.

So presoak the cukes if that is what your recipe calls for. Standard recommendation is to begin with fresh syrup to eliminate the molds and bacteria that have developed in the existing syrup over time.

If for some reason you feel you must use the existing syrup even though it isn't recommended then yes, you will definitely want to reheat it first.

Dave

If it's a high-vinegar high-sugar syrup, then by boiling you could probably get away with reusing, but one problem is the cucumbers continue to leach liquid into the syrup, reducing the acidity and making it more susceptible to further contamination as fresh cucumbers are added.

It's your call.

Carol

My experience is exactly what reading lady suggested. My family will literally eat a half pint the day it is made. I have tried adding cukes to the brine (and they will be gone by the next day). I can get away with this once, though you do notice a difference. Can't do it more than once.

My brine is sugar, vinegar, salt, and spices (no water). The cukes lose a lot of water and the brine is diluted. I do not worry about molds since they are eaten so quickly. I would not add more if they sat in the fridge for a while.