Should i change my main ?

Hello everyone, im plat 3 Vel'Koz(former) main with over 400k and i am looking to retire vel koz and go for Cassiopeia. The reason for this is that the Vel'Koz is the only champion i know how to play in every situation so when he gets picked/banned which is a rare ocassion i pick xerath maybe orianna but i am not even 20% good with them as i am good with Vel. So after 2 years of maining him i am looking to change it to cassiopeia because she is insane scalling ap carry on mid lane and has interesting kit. But i am not sure if it is too late for me to do something like that bcz i was playing vel koz since i was bronze 4 back in season 7 . Its not like he is boring or something its just the fact that i dont know how to play any other champ at the decent level and i am a bit tired of being OTP in rankeds. And i think i dont deserve my elo cuz i abused pretty broken control mage to get to platinum . By retiring vel koz i meant i will still use him but as a secondary pick or a counter pick