This script updates prices for up to 100 customers at a time. There IS manual config to do after running.

stripe.js

const stripe = require('stripe')('put your stripe secret key here');

const stripeFunction = async () => {
  const subscriptions = await stripe.subscriptions.list({
    limit: 100, // The max limit is 100, so you can do 100 at a time; change to whatever number you need to change
    price: 'replace this with your price id' // This is the price id you want to replace
  });
  const count = subscriptions.data.length; // this is an optional line, just counts how many customers the Stripe API returns
  subscriptions.data.forEach(async (data) => {
    console.log( // Log update subscription object
      await stripe.subscriptions.update(
        data.id, {
          cancel_at: 1606888800, // REPLACE THIS: Unix Timestamp (Epoch Time) on when to "cancel" it.
          proration_behavior: 'create_prorations', // To prorate, or to not; replace with 'none' if you don't want proration
          items: [{
            id: data.items.data[0].id,
          price: 'price_1HtRZ1D9X4XcXS1qi1lVnAC0' // Price ID you want to change it to
        }]
      })
    );
  });
  console.log(count); // Logs the number of users returned; intialized on line 8
}

stripeFunction();

// IMPORTANT: You will need to manually click "Don't Cancel" in the Stripe console for each user