Other than the day they arrive I don’t weigh any of my snakes or their prey anymore. I’ll admit I did do this 10 years ago but I quickly learned that it’s totally unnecessary when it comes to feeding. I understand that breeders have to make sure their females are a certain age/size/weight for breeding purposes.
When it comes to prey size, my motto is, “if it fits, they gets.” I typically feed my snakes prey that’s 1-2x the widest part of the snakes body. I’ve seen 125g pythons easily smash a 40g jumbo mouse. It’s ok to feed larger meals as long as they’re not outrageously oversized. You’ll just need to increase the intervals between feedings. I’m sure your BP can handle rat pups. Just feed the smaller ones first if you’re uncertain. I’d be more concerned with changing prey type then prey size especially with a BP. Some of them can be picky and stubborn. I hope he transitions seamlessly for you.