Not to be that guy, but do you have a source for this? As scummy as what they're doing is, it's also completely typical for damned near 100% of companies to reserve the right to be scummy. They all have a clause in their terms that states, 'We can do whatever we want, whenever we want, for any reason we want. If you don't like it, your only recourse is to stop using the software.'
I'd love for this to be true, but I've never seen as exclusion or other sort of work-around for EU customers in any sort of terms for this exact condition, so I'd be extremely surprised to find out it's really not allowed in the EU.
"Unfair contract terms" its pretty easy in most cases especially if its something as blant as changing price backward without acceptance of other site.
Also every country can have different interpretation (It is possible that some countries are very limited and you have to try EU level help) In some cases its even illegal to write non-competition into your contract without providing payment for the time of this being in-place ( there were tons of cases when contract was voided by that and I have even one of my contract ended on my account after single lawyer visit cause of this :] )
I would assume Unity won't even try to enforce it in EU.
But ofc. after update its over and games not yet released can most likely be forced to pay.
Probably dependant on country interpretation, but you can try make it to EU level institutions for proper help.
It seems to be pretty easy to get your contract to terminate here in Poland even in case of non-competition clausule so it would be even easier for something like retroactive price change.
I'd love for this to be true, but I've never seen as exclusion or other sort of work-around for EU customers in any sort of terms for this exact condition, so I'd be extremely surprised to find out it's really not allowed in the EU.