My wife has an Extension until November 2019, piggy-backing on mine. Her Lao passport is almost full. If she gets a new passport in Laos, does she then just show both her old passport with her Extension stamp and Re-entry permit, and her unused new passport, at the airport in BKK? Then get the stamp/permit transferred at Chaengwattana?
TLDR : Answer Summary
If your wife obtains a new passport in Laos, she can show both her old passport containing the Extension stamp and Re-entry permit, as well as the new passport when arriving at Bangkok airport. It is advisable to transfer the stamps and permits at Chaengwattana in a timely manner. The transfer service is free, although the Laos government may handle invalidated passports differently.