Amy,
I understand that adding sand from other systems could enhance the bio-diversity in your system.
You mentioned that you are using the sand from your established tank.
Over time the sand and rock in your tank losses it's alkaline reserve, calcium, magnesium and strontium. A thought to keep in mind is boosting these elements that are needed in your system that get depleted by new sand and rock.
IMO when moving into the 125 might be a good time to add new sand.
Just a thought ...maybe consider doing a mix of some sort.
Linda