Changes like this are never going to be popular, but Riot have chosen to increase the cost of Riot Points to their european players. Basically it amounts to about a 10% increase in the euro equivilant of RP purchases. With ten euros getting you 1780RP, after march 20th it will now only get you 1575RP.
Riot is huge now and as they admit, most of their trading is in US dollars. So the price of currency in relation to the dollar is going to always be in their interest. But you can also be quite sure they would be unlikely to change the prices back should the economy start to go the other way.
Prepaid cards will also take a hit so you have until March 20th to redeem any RP cards if you play on the European servers. You can read Riot’s full public announcement and get more details here.