It's a couple of steps but can be done. You may want to reconsider getting off of LMS since the latest Prime Infrastructure (PI 1.2) does not have full feature compatibility with the latest LMS (4.2.3). Have a look here for the comparison.
If you still want to do it:
1. Upgrade your NCS to PI 1.2 (instructions). If you're going with a total of 550 devices on a combined license you should use the medium ova file to get the correct server resource allocation. You will need to get the new license from the TAC or via the licensing portal. I'm not sure if you can get the PI license for your LMS upgrade directly via the portal but you do have the entitlement for LMS 4.2 and PI 1.2 (combined licenses not to exceed the original LMS number) if your SAS is current. You may need to contact the TAC and ask for the licensing team.
2. Upgrade your LMS to 4.2 and patch it to 4.2.2 (instructions). Once you've done that, there is a simple menu command to export your device credentials to Prime (Admin, System, Export Data to PI in LMS and then Operation, Device Work Center, Bulk Import in PI - instruction on Page 18 here). Do that and then in the new PI installation import the devices.
Verify everything is working and you can decommission the LMS server.
