Mike,you wont have a cycle just by adding sand.Just make sure that when you are moving the sand from the 90 over to the 125 gallon that there enough water to cover the sand.I kept mine in a couple totes and occasionally rocked the tote back and forth every 10-20 minutes.Make sure the sand doesn't get cold as well.
Don't be afraid to rinse the sand with old tank water to get some of the detritus out.
Make sure you have plenty of water mixed up before hand.I moved my 90 gallon into the same place my 55 gallon was,added extra sand and even had to take an hour to do some plumbing.All in all it took 3 hours.Never even had the hint of a cycle.
I would suggest when you add the new sand add it as to not cover the old sand.