Question: Will the serial interfaces on the routers still exist ? (ie: are you swapping out the routers)
I ask, because if the routers stay the same but the serial interfaces have no traffic you can just leave them monitored and unmanaged to some absurd time in the future like Jan 1 2038 (unix epoc rollover date). This will show the point in time when the serial interface became inactive and the ethernet ports on the routers will now start to show traffic.
If at some point in the future you are asked for stats comparing before / after, you can pull the raw data for the old serial interface into excel and then merge in the new ethernet stats. This would cover a lot of bases depending on what kind of report people ask for ( %util, availability, min/max/avg tx/rx)
Possible problem: If you have alerts or views that would include the old serial interfaces, you'd need to update them to exclude the serial interfaces.
If the routers are being swapped out, then ignore everything I just said. Exporting whatever you need from a reporting POV may be the simplest considering you're dealing with hundreds of ccts.
Side note: remember to match use the custom properties on the interface to set the upstream bandwidth into the MPLS cloud. Otherwise it will look like the utilization of a GigE interface is trivial when it is being throttled by an upstream limit of 150Mb
Chris