Good advice all through the thread.

Work on the rat transition now and keep it going. I have a coastal that is somewhat picky although she isn't mouse picky just has her own style of eating.

I would add another option. Like Reinz said combining mice and rats in the same bag for a scent transfer is an option, you could also try the same with chicks or quail.

All Morelia/Carpets are highly arboreal, and any type of avian prey will usually excite them. I'm not suggesting feeding quail or chicks outright, but scenting a rat with some type of avian item should be like "adding gravy to mashed potatoes". It may be just enough to get the snake interested.

First wait it out at least 2 weeks and get the snake in the hunting mode. Once you get it going it should be an easy conversion.

After a while, you should be able to mix rats and quail every couple of feedings and have a steady feeding situation.

You could also start with a mouse, and "piggyback" a rat when the mouse is almost down.

These are a couple of options.

My coastal was never on mice, but when she turned her nose up at rats, I tried a live one and she went nuts.