Any way to change which portal a collector is associated with?

I'm not sure if there is a way to reassign a collector to another portal, but it would not help you in a migration anyway. The devices and other information is not stored on the collector but in the portal itself.

You'll have to rebuild the devices into the new portal. You can make it easier by using the API and scripts that can export data from one portal and use it to populate the other portal with another script. Perhaps LM support can get you something to help with it.