There was very little between the two sides in the first hour, with Japan threatening to claw back their 7-0 half-time defecit.
Keira Bevan had slotted in at 10, but struggled to put Wales on the front foot with her kicking game.
Lynn instead introduced Lleucu George, with Bevan reverting back to scrum-half and Seren Lockwood moving out to the wing.
It was a reshuffle which paid off with George’s boot putting Wales back in the right areas, and that coupled with the introduction of experienced forwards Gwenllian Pyrs, Bethan Lewis, Gwen Crabb and Georgia Evans proved the turning point in the match.
“It was a good result,” said Lynn.
“Going in at 7-0 we knew there were areas that we needed to improve and I could see on the 53/54-minute mark that we needed some more territory.
“The girls, all 23 of the squad, were outstanding, and I thought the bench made a really big impact which was really good to see.”
