The deals change monthly or weekly and occasionally they do have promotions that are geared towards existing customers who don't want to trade-in their previous device or add a new line to qualify for the promotion. For example, I got my current phone under one such promotion and it was June.

The biggest thing existing customers should realize is those promotions tend to not be for the latest and greatest or newest phones. For me, I had no interest in newest iPhone or Samsung phone, but I was able watch and wait for the right deal for the best phone that fit my needs and budget.