Moving an AD user account from one domain to another within same forest
Solution 1:
Having just tested it, I can vouch for the fact that Move-ADObject can move objects between domains. The syntax you need is as follows:
Move-ADObject -Identity "CN=Test Move,OU=SourceOU,DC=Domain,DC=suffix" -TargetPath "OU=Test,DC=child,DC=Domain,DC=suffix" -TargetServer "Target-Domain-RID-master" -Server "Local-Domain-RID-master"
This moved a user from the forest root to a child domain.