Because of the config, specifically the 'telephony-service' config, I needed to keep the same flash I had in the original UC520. Why? Because the phone loads that were in the config were on that flash. Not to mention the IOS for specific features that may have been needed. So, with that said, the first thing I had to do was to get that original flash out and put it in the replacement UC500. No big deal, you just have to think about it.
NOTE: When taking the screws out to get into the inside of the unit, there is one embedded screw behind the faceplate that will drive you nuts. You cant see it until you get the faceplate off. See here to the left.
The third thing I had to do was to change some of the config in the ephone-DNs and ephones. Why? Again, the user licenses for ephones and ephone-DNs. I was only allowed up to 56 ephone-DNs with the replacement, and only 14 phones total (even though it was an 8 user license) with the replacement unit. So, ephone-DN and ephone numbers had to be changed (i.e. 'ephone-dn 65 dual line' had to become less than 56 / 'ephone 28' had to become 'ephone 5'). I had to get the customer to select what phones they wanted to keep and what phones they wanted to do without (again, because of user licenses) and number them appropriately (according to what the replacement would allow). No big deal, just had to work though that. This took some time though. I also had to sacrifice some phones due to ATAs that were needed for credit card machines, faxes, and security alarms. No big deal, but still had to account the ATA count in the ephone number allowed.
Physically replacing the unit was not a big deal (although I was reaching above my head while standing on a rolling chair - NOT fun). I just labeled the wiring and where it went and matched it up with the replacement. That saved time since I didn't have to remember anything. I just looked at the cable.
One thing I forgot to mention was that I had to take out the T1 module out of the unit that went bad and put it in the replacement unit. I didnt have one, so I had to use that module.
Anyway, just some thoughts on my experience on changing out one UC for another with different licenses and hardware.