Changing rival starter pokémon in red and blue

[P_k1ba]

Hello! I’ve used PKHeX before, but I still don’t fully understand how all of its systems work. I’m P_k1ba, and my goal is to clear every Pokémon game while putting in as little effort as possible.
(I don’t cheat by generating items or boosting Pokémon levels — that’s not what I mean.)

My problem is with Generation 1. Pokémon Red and Blue can use each other’s save files without any issues, but the rival’s starter Pokémon stays the same as the one chosen in the original version.

For example:
In my Red save file, I picked Charmander, so my rival uses Squirtle.
However, when I load that same save file in Blue, I want my rival to use Bulbasaur, because I plan to use Squirtle for this playthrough.

Is there any way to change the rival’s starter to Bulbasaur when using my Red save file on Blue? I hope someone in this community can point me in the right direction. Thank you!

— P_k1ba