Train ticket cancellations

Hi everyone,

I've got a super off-peak return journey booked from London to Bristol Temple Meads this Saturday, however plans have changed and I'm now going to be in Birmingham the night before and will want to travel to Bristol from there instead. I've booked the train from Birmingham to Bristol, but I'm wondering if its possible to cancel my return journey from London, get a refund, and re-book the "return" (bristol to london, the day after) seperately to save money? The new rules on refunds are confusing me, and I'm struggling to find information on google not relating to your train being cancelled or delayed. I thought you lovely people might know the answer. I booked through LNER via the app if it matters.