Yes its easy, just purchase the XM unit from Infiniti and replace the unit in the trunk; Our have your dealer do it; The car is built for either unit and no issues should exist.
There is likly even an after market for the old unit if you wanted to sell it.
The only part that I'm not 100% sure about is the Antena connectors as I ordered mine as a service part. Joe at
http://www.infinitypartsusa.com was VERY helpfull when I ordered my unit and had it shipped to Canada. I trust that he would be pleased to assist you with this switch and some advice. They are an Infiniti Dealer so you will be getting the true OEM parts also.
Anyway, I'm sure that if you wanted to, changing to XM will be no big issue, other than the cash to buy the new unit. I think the kit was around $500 US.
tk