If I sign up for the introductory offer for the Core plan, will I be able to upgrade to the Creative plan at the reduced introductory price?

If you upgrade during the introductory offer period, you'll pay the reduced price.

For example, if you sign up for the Core plan on June 1, 2020 and upgrade to the Creative plan 40 days later (on July 10, 2020), you'll still pay the reduced price.