Are old users eligible for introductory pricing?

Solution 1:

Probably, yes. Given the quote from Apple's documentation below (emphasis mine):

Based on the receipt, you will find that new and returning customers are eligible for introductory price or free trial discounts as follows:

• New subscribers are always eligible.
• Lapsed subscribers who renew are eligible if they haven't previously received an introductory price for the given product (or any product within the same subscription group).

However, do ask App Store Connect Inquiries directly. They will be able to provide an absolute and authoritative answer.