Membership Renewal Reminders not Sending for Upgraded Memberships
The "join date" is the "Member Since" date; it's a bit odd that there's no option to select "Membership Start Date" in Scheduled Reminders.  Perhaps someone else could explain what I'm missing. 

You could work around this via CiviRules, or adding the Start Date to the Scheduled Reminders would be simple.  It's SO simple that I'm sure there's a case I'm not thinking of why that's not desirable...

Also - you can create a reminder on the "Member Renewal" activity type instead.