The fare rules partially for international have many restriction, so the return must start from the region of the original destination.

Domestic is likely to be a bit more flexible but will be restrictive to a similar domestic leg and not a unrestricted FFC. It depends on the exact fare rules of the original ticket that restricted the return, To use the change, this will be priced as a change of the original ticket. Fortunately domestic restrictions are less and some case allow repricing with out repricing the outbound.

You will need generally to work with an agent and don't be surprized if certain choice led to outrageous outcomes.

For domestic it is best to book as separate oneways and you avoid this issue.