"She smiled when the dog came home."
Translation:Hun smilte da hunden kom hjem.
No, they are not interchangeable, but it's common in some dialects to use 'når' in cases where 'da' would be used in Bokmål.
"When I was at the store"
"Da jeg var på butikken" or "Når jeg var på butikken"
"When I grow up/When I get big":
"Når jeg blir stor" != Da jeg blir stor"(incorrect in text and speech)
Also, da/når do have several different meanings, and it's only in certain contexts where 'da' can be replaced by 'når'.
"I'll do it then"
"Jeg vil gjøre det da" != "Jeg vil gjøre det når"(incorrect in text an speech)
I'm just pointing out in case someone ever hears anyone say this and can be prepared of its ambiguous meaning.