Realistically no. Also demanding retroactive rewards when a QOL change happens creates negative feedback when a positive change is made and does not encourage companies to keep doing said changes.
Please understand there is no guarantee for stability over any "products" Riot offers. A skin may change, get a new splash, champion may be reworked. Loot boxes are RNG inherently, if you got good stuff out of them in the past that's the reward you got you don't retroactively get any reward for it. Its not unfair because it applys the same way to everybody at the present. If there are differences between say what loot you can get biased off your experience level that would be unfair.
You may try to send a support ticket over a OE reimbursement if you have a recent upgrade but odds are they will point to the terms and conditions you agreed to when purchasing RP or EULA.
Its not exclusive to riot, sales happen promotional evens happen, prices rise and drop. You would never have any luck going to a burger joint that dropped their prices and demand they reimburses ya for previous purchases. No one thinks its unfair they don't do this, what exactly makes you think its unfair riots changed their product to be more cost friendly going forward which only serves to benefit you and everybody else and some how your owed for buying their "burgers" in the past?
Tldr. Your not owed anything beyond what you spend RP BE OE on, if you don't like the value of your purchases at the time don't do it. If or when it changes that's an inherent risk you should have been aware about on your end.