Couple of things.
Your talking about measing the wings height above the road. This is not how wing movement is measured. Wing movement and positioning (and positioning of pretty much everything on an F1 car) is taken from the reference plane, not the ground. Measuring from the ground is no good.
And testing parts from previous races against new rules not only means retrospectively applying rules that didn't exist (which is just stupid, let alone unfair), but also assume the FIA would know what they are likely to change in the future to know which parts to keep, unless you suggest that a new car should be built for every race and every car should be locked away in an air tight room as soon as it finishes the race?
The RULE hasn't changed, it hasn't changed in the slightest the test to catch people breaking the rule has changed, which is completely different.
IE one race the rule is you can have 40mm flex, the next its 20mm, applying the 20mm rule to the previous races 40mm rule is nuts.
The rule WAS 20mm for the first race, the last race and this race. If a wing breaks the 20mm in every race up to the new race, where the TEST becomes more accurate, but the rule is the same, and the teams put on a different wing that doesn't break it but the old wing would fail. Testing the old wing would merely prove they WERE breaking the existing rule previously.
Passing a test does NOT make something legal, it merely means they can't prove its illegal. Same way being found not guilty of a murder is only stating there isn't enough proof to find you guilty, you can absolutely do something illegal and get away with it.
Its simple the rule is 20mm of flex, this rule hasn't changed, creating a wing that won't bend more than 20mm under the specific test, but absolutely bends more than 20mm in real use, when the rule stats it can't bend more than 20mm is breaking the rule, just not getting caught.
Ultimately for less cheating, we need better rules. The rule should be, it must pass testing so it doesn't bend more than 20mm, but it CAN bend more than this in the race. The 20mm rule is to help reduce the amount teams do it basically. With no rule they'd make crazy bendy wings, if they say it can't bend more than 20mm even the best wing they can design will almost certainly bend less than one that also has to pass the test. If they really intend for the wing to bend no more than 20mm, make the test 10mm or something, so while they will try and design it to bend further in real use, the limits of the test make it more likely the wing will bend less.
Basically they make too many rules with poor tests and should account for that more.
As long as the rule is CLEAR that it must pass but it can bend more in use, then a team with a 130mil budget can try and create a bendable wing, while the rule is that is SHOULDN'T bend more in real use, a team with a small budget can't risk developing such a wing, because if its deemed illegal because the tests change, they've lost that money. This is where Ferrari/Mclaren/RBR's huge budgets come into play, investing in the little things that all add up to make the car better, but they don't sacrifice elsewhere.
Basically we've got a grid with the richest teams making parts that do break the rules but pass the testing and other teams that can't afford to even consider trying it. A more realistic rule is fairer for everyone.