If you have a SA license and keep it current, you dont need to re write any exams. If you let it lapse for an extended period then you need to rewrite, if more than 60 months you have to do all the theory from scratch.
In the case of a validation, they are valid 12 months. After it expires you need to do another validation. Same ground rules apply, air law and flight test. Then you got it for another 12 months. You cannot validate in light of holding a previous validation.
Rather than validating you must convert. You will write law, performance & planning and also Met. Once you have done these you have 36 months to get your license issued via flight test or you need to do them again as well. Once you did a convertion, you won't have to sit all exams again.
Before trying to convert make sure to check the AIC for requirements. Once you have converted, you will be issued with a SA license on the bases of a license held from a contracting state (ICAO).