convert_extension.js

const path = require('path');
const fs = require('fs');
const { promisify } = require('util');
const glob = promisify(require('glob'));
const FROM = '.css';
const TO = '.scss';

const mv = async file => {
    const out = `${path.join(
        path.dirname(file),
        path.basename(file, FROM)
    )}${TO}`;
    const buf = await fs.promises.readFile(file);
    await fs.promises.writeFile(out, buf);
    await fs.promises.unlink(file);
};

(async () => {
    const files = await glob(path.resolve(__dirname, `**/*${FROM}`));
    const promises = [];
    for (const file of files) {
        promises.push(mv(file));
    }
    Promise.all(promises);
})();