Candace, Ferb and Buford are the only crossover characters from Phineas and Ferb who didn't voice other new characters in Milo Murphy's Law before The Phineas and Ferb Effect while Phineas, Doofenshmirtz, Monogram, Perry, Baljeet, Isabella and Irving's voices have all played original Milo Murphy's Law characters before they appeared in this crossover. Melanie, Albert, Jenny and Paul are other Phineas and Ferb characters who do not appear in this crossover, but their voice actors have all appeared in this crossover or other episodes as original Milo Murphy's Law characters.
While Dr. Doofenshmirtz, Major Monogram and Perry's voices already voice sub-main characters in Milo Murphy's Law, as they did in Phineas and Ferb, the crossover voices of Phineas and Isabella also play recurring Milo Murphy's Law characters while Baljeet and Irving's voices guest starred in Love Toboggan/The Island of Lost Dakotas (2017).
Although This Episode Premiered In 2019 It Was Made In 2018 According To The Credits