Plan upgrade screen in migration flows doesn't show pro-rated pricing

[daledupreez]

Quick summary

The plan upgrade screen that we show in our migration flows always shows the generic price for the Creator plan. However, this ignores any proration that may apply for sites where the user has already purchased a lower-tier plan. They should see the price to upgrade, rather than the full price.

Steps to reproduce

1. Set up a site with a valid, paid Explorer or Starter plan
2. Navigate to /import/:siteSlug or to Tools -> Import in the left nav for that site
3. Enter a valid WordPress site in the text field and click on the CTA to proceed
4. Choose to migrate your content
5. On the plans screen, you see the full/standard price for the Creator plan, instead of the upgrade price
6. Click on the "Upgrade to migrate" button
7. Verify that you're shown a reduced price in Checkout

What you expected to happen

The plan upgrade screen should show me how much it will actually cost me to upgrade if I have already paid for a plan.

What actually happened

I see the full price on the screen, which is more than I would actually pay.

Impact

Some (< 50%)

Available workarounds?

Yes, easy to implement

Platform (Simple and/or Atomic)

Simple

Logs or notes

No response