Going on experience on five other systems that have been rebanded, the first "actual" noticable changes have been within 4 months of the YE grant. Finalized changes have taken anywhere from 6 months to an year depending on issues that may crop up. Usually within 6 months if they are not doing an major overhaul, the system is fully operating on the new freqs. I have seen one system that was fully operational on rebanded freqs ONE 1 month after the FCC YE grant. They had been working for many months before hand to make changes using an STA grant, and when they recieved the final permanent grant were ready to go.
