Oracle Global Human Resources Cloud supports two-tier employment models: single assignment (SA) or multiple assignments (MA), with or without contracts (SC or MC). The employment model is set at the enterprise or legal employer level via "Manage Enterprise HCM Information" or "Manage Legal Entity HCM Information." Changing this model post-implementation is possible under certain conditions.

Option A: Correct. Oracle allows flexibility to change the employment model (e.g., from two-tier MA to two-tier MC SA) at any time, even with existing work relationships, as long as data migration and configuration adjustments (e.g., contract setup) are handled. The system does not lock the model once set.

Option B: Incorrect. The client can transition from two-tier MA to two-tier SC SA post-implementation, though it requires updating existing records and ensuring compliance with new contract rules.

Option C: Incorrect. Within the same legal employer, only one employment model can be active at a time. Mixing MA for existing employees and MC SA for new hires in the same legal employer is not supported without separate legal employers or a model change.

Option D: Incorrect. Contracts can coexist with multiple assignments if configured as multiple contracts (MC), so the model can be changed even with existing employees, contradicting this statement.

The correct answer isA, as per "Implementing Global Human Resources" on employment model flexibility.