Cannot redeem lifetime offer

I received a promo code by email for a lifetime Plex Pass for $74.99. When I click on the link it says: “We like you too much to take your hard-earned money twice. User NickC4555 already has a Plex Pass subscription.” The options are: Switch User | Change Plan | Redeem Promo. Redeem Promo is for allocating a discount retrospectively to an existing plan, switch user isn’t what I want to do so I click Change Plan. That takes me to the upgrade to lifetime plex pass, but when I enter the coupon code it says “Code not valid”.

Any suggestions, please?

To my knowledge the offer is only valid for new Plex Pass subscriptions. You are already subscribed.

The subject line of the email I received is “From us to you: A limited-time offer for long-time customers”, so if it is only for new subscribers that is misleading.