Do not wait. Switch over as soon as possible because the two services work together rather than replacing one another.
InmateAid does not replace your GTL account. Your GTL account stays open and the balance remains available. What InmateAid does is provide a different number for your inmate to dial that triggers a lower rate on the GTL system. The calls still run through GTL and still draw from your GTL balance, but at a significantly reduced per-minute cost because of the local number being dialed.
The practical effect is that your existing GTL balance stretches further immediately. Instead of burning through a $20 deposit in a certain number of calls at the standard rate, that same $20 produces roughly three additional calls when your inmate is dialing the InmateAid number at the lower rate. Every dollar already in your GTL account goes further from the moment your inmate starts using the new number.
There is no reason to wait for the balance to run down first. Every call made at the standard rate before switching is money spent at a higher cost than necessary. The sooner the InmateAid number is in your inmate's hands and on their approved call list, the sooner the savings apply to every call including the ones drawing from your current balance.
Get the InmateAid number set up today, send it to your inmate through a letter as quickly as possible, and let the lower rate go to work on whatever balance you already have sitting in the GTL account.