Flex Plus means you can change for free, paying the fare difference. There's no cost for the process of changing itself, unlike Flex and Saver fares where on top of the difference in fare and taxes you also have to pay a change fee.

Upgrade availability has no link to whether you buy Flex or Flex Plus, Flex Plus just makes upgrading cheaper in terms of miles (and cash usually) cost. It is frustrating not finding availability but if one monitors closely generally something opens up at some point. So far I've always been able to spend my miles on upgrades or award tickets and usually have too few miles for an upgrade as when I have enough I use them. Just getting back into the region of having enough for another upgrade (at 35k now, will hit 42k in 2 weeks)