Our company plays the "switch carriers every 2 years" game, depending on which carrier offers the best deal. Switching numbers is never a problem, everything was pre-programed and in a matter of minutes 100 phone numbers were moved over.
Our issue was about 20% of the Motorola phones and about 25% of the iPhones were defective right out of the package and didn't work. Verizon rep was on site for 4 hours, but had no spare phones, everyone with a defective phone had to go to the Verizon store to swap out the phones. We are switching again in 2013, back to Sprint. Bosses finally got smart, contract specifies that Sprint will have a rep on site for 24 hours on switch day, with a supply of replacement phones so defective phones can be switched out on the spot.