migrate.js

const github = require('octonode')
const repoName = /* repository name */
 
const client = github.client(/* user token */)
const repo = client.repo(repoName)
 
async function migrateAll() {
    let page = 1
    while (true) {
        const prsResult = await repo.prsAsync({page: page, state: 'open'})
        const prs = prsResult[0]
        page = page + 1
        if (prs.length === 0) return
        for (let i = 0; i < prs.length; i++) {
            if (prs[i].base.ref !== 'master') return
            console.log('Migrating ' + prs[i].number)
            const ghpr = client.pr(repoName, prs[i].number)
            await ghpr.updateAsync({'base': 'main'})
        }
    }
}

migrateAll().then(console.log).catch(console.error)