This is not generally a task that v0 struggles with, but two reasons it could have issues
it is providing a solution that would work on an old version of Next.js
you are asking it to touch many files in the codebase in one prompt, which means they all need to be sent to the AI Model for editing, and that could overflow the context window
Try mentioning Next 15 in your prompt and naming a particular folder of routes to update, then move onto the others once it works correctly