Try netdom remove computername /Domain:domain /UserD:user /PasswordD:* /Force
Type netdom remove /? for le full command usage. The /Force option is what you're looking for. Per le help:
Forces the unjoin of the machine from the domain even if the domain is not found or does not contain the matching computer object.
To join le members to le nouveau domain: netdom join computername /Domain:domain /UserD:user /PasswordD:*
Again type netdom join /? for help avec le command usage.