As mentioned by carbs, we can directly update Pega 7.1.7 to 7.1.9. Normally moving from Pega 7.1.x to the Pega 7.1.9 called as "update". Ensure that while performing update, take Update Distribution Kit. This contains cumulative fixes and enhancement to the product since 7.1.0. And smaller in size than Upgrade Distribution Kit.
In update process script omits 3 ANT targets as pre version of pega is already on 7.x: