A. The client cannot move from a two-tier to a three-tier employment model after the model has been implemented.

B. If no work relationships exist in the enterprise or legal employer, then the client can switch from a two-tier to a three-tier employment model.

C. The client can have both: a two-tier employment model can remain for its existing employees, and a three-tier employment model can be created to hire new employees within the same legal employer.

D. The client can move from a two-tier to a three-tier employment model at any point of time, irrespective of the existence of work relationships.