How were they able to keep Rick's voice?

Voice acting, it’s not uncommon for multiple people to voice a character throughout the life of the character.

https://www.theverge.com/23919126/rick-and-morty-voice-actors-ian-cardoni-harry-belden

There are like a million people who can do a pretty good Rick voice, and half of them can also do a pretty good Morty voice, and a few of them are voice actors.

(Though it looks like there are two different voice actors for them now.)