I upgraded from 2 units about 5 years ago. I just disconnected them and connected the 2 new units before calling in for a radio swap. About 8 months later I got curious after hearing about others who had units who missed their signal and connected them. Sure enough they worked and still do to this day. About 11 months ago I disconnected the 2 units I was paying for then called in to cancel the subs on them in which they never got deactivation signal. They even get their updates when they are pushed out and continue to work fine.
I'm not sure exactly what the time frame it's on when they stop sending the signal. Forums around the internet say about 3-6 months at intervals of once a week is when it is sent then after that it stops sending that signal. I would at minimum wait until 8-9 months pass by to be on the safe side.
These are also Sirius radios as well and not XM units. That also could have something to do with the ease of missing their signal for deactivation. --
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