Quote from the v9.6 release notes:
"...any Limited Series will be freely available to start from the day they unlock until the end of the Round."
Update: Turns out this series was an exception to this rule. It's a "Replayable" Series that doesn't expire even after you've completed it. Highest PR needed for #15.3 is 77.8. This means that 25 of 28 upgrades will allow you to complete the series.