You can change the cardfile on a machine (Asset) or contract at any time, however you cannot change it if you charge minimum volumes.
This is because if there are certain clawbacks set up, sometimes the unders are held and are billed in the future.
If the unders were originally billed to the prior cardfile and they need to be clawed back the job will be for the wrong cardfile
Further information
Assets Swaps and Early Billing