It is not an "upgrade" like software. It is two totally different systems. Sirius is a system that uses three 4 MHz carriers on the S-band at 2320 to 2332.5 MHz. It decodes the signal using a proprietary algorithm contained as hardware within the radio. XM uses six 4 MHz carriers with a different latency speed on the S-band at 2332.5 to 2345 MHz and uses a software based decoding algorithm from Coding Technologies.
In short it is Beta-VHS, Diesel-gasoline, etc. Two totally different systems trying to achieve the same thing. The only way to upgrade a Sirius radio or and XM radio to a SiriusXM radio is to remove it and replace it with one that is set up for both signals. Which would cost a lot of money and, bluntly, once you remove the dash of a car, it is never the same again.