Rebanding
Actually, Nextel is adamant about not paying for upgrades. They will replace one for one if your radio will not work on the new frequencies. You may have a GTX today...Nextel will replace it with an analog only XTS1500RB. If you are analog today, unless you (not Nextel) pay for the digital upgrade, you will be analog after rebanding.
As mentioned, your radio may be replaced with a newer "rebanded" version, but it is incumbent upon you to add any features beyond your current operating characteristics. Agencies may take the opportunity add $1k flash upgrades to those "RB" radios if they would like, but that is outside of the scope of rebanding.
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