He made the switch to a new woman as soon as he was dumped by his old girlfriend.
Shortly after his girlfriend broke up with him, he rebounded with another girl.
ーHe made the switch to a new woman as soon as he was dumped by his old girlfriend.
「彼は元カノにフラれるとすぐに新しい女性に乗り換えた」
to make the switch to で「…に乗り換える」
to be dumped で「フラれる・捨てられる」
ーShortly after his girlfriend broke up with him, he rebounded with another girl.
「彼は彼女に振られた後すぐ、失恋の悲しみを紛らわすために、他の女の子と付き合った」
shortly after で「〜してすぐ後に」
to break up with ... で「…と別れる」
to rebound withで「失恋の悲しみを紛らわすために…と付き合う」
ご参考まで！
カナダ 
