Page Router Projects to Next.js 14+ Issue

[fecommunity]

Link to the code that reproduces this issue

https://github.com/fecommunity/reactpress/

To Reproduce

Clone ReactPress: git clone https://github.com/fecommunity/reactpress

Current version: Next.js 12.3.4 with Page Router

Attempt upgrade to Next.js 14.2.5 following official guide

Observe multiple warnings and broken functionality in:

Dynamic routes with getStaticPaths

Custom _app.js and _document.js configurations

API routes using Page Router structure

Current vs. Expected behavior

Official backward compatibility for Page Router through Next.js 15

Automated codemods for common Page Router patterns

Hybrid routing documentation for gradual migration

Provide environment information

Repository: https://github.com/fecommunity/reactpress

Pages: 200+ documentation pages

Key Features: SSG, i18n, advanced routing, custom docs structure

Current Next.js: 12.3.4 → Target: 14.2.5+

Which area(s) are affected? (Select all that apply)

Error Handling

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

// Current ReactPress page structure (needs migration clarity):
// pages/docs/[[...slug]].js
export async function getStaticPaths() {
// Complex i18n + documentation path generation
}

export async function getStaticProps({ params, locale }) {
// MDX content loading with i18n
}

export default function DocPage({ content, toc }) {
// Page Router component - how to migrate to App Router?
}