They could stick the old machines on domestic as they will be relatively low cycles, however it means they will need to remove the old interior and put in new domestic spec seats galleys etc. The cost of this may be worth it in the long run especially if the work coincides nicely with the old ones needing checks. The issue though may be the lack of brake cooling in the older machines, plus they were looking at HUDs in the new ones to help with RNP AR approaches as well as other little things.
Either way some of the new aircraft will end up on the domestic service as they getting 14 new and there are currently only 12 doing the internationals.