So I have two options (that I can think of.)

Option 1:
Offer Featured Pacakge(s)
Standard (Free)
Featured (Paid)

If someone chooses Featured when signing up, or if they post as Standard and then use the Renew-Upgrade button to change to Featured, their listing gets Featured, the date gets updated to the current date when they pay for the Featured Package.
Drawback: if featured period < standard ad expiration timeframe. When the Featured time period expires, a featured listing just disappears totally from the web, which can be bad for the site's SEO and which they may not like. For example, if they're not paying attention for a day or two when the featured period up for renewal, their listing totally disappears and they miss those days totally until they notice they need to change plan or pay again for featured. If they had not paid, their ad would not have disappeared (if the standard expiration time is > featured time period)

Option 2:
Standard (Free)
Upgrade to Featured (Paid)
If someone posts as Standard, then Upgrades to Featured, listing gets featured. Post date does not change, but when Upgrade to Featured period expires, ad continues to run as standard ad.
Advantages - no surprise invisible if they don't notice the exact day it expires
Disadvantage - if they use the upgrade/renew button, it wipes out the (paid for) featured status; Posting date doesn't change with Upgrade to Featured so they may use the upgrade/renew button wiping the paid featured status out.

So it looks like I could request one of two customizations in the short term:
1.) Change status to active/standard at expiration of active/featured package
2.) Change post date when Upgrade to Featured is paid, and eliminate separate upgrade/renew button