I am new to RISC-V and compared to other ISA’s from what I have seen there are not as widespread of forums and online discussions. I recently learned how to add a new custom instruction into the RISC-V ISA but I wanted to know how could I modify already existing ones? For example if I have the sub instruction if I did:

a1 = 5
a0 = 10

sub a0, a0, a1

Instead of returning 5, I want to change sub to instead add and return 15 (yes I am aware of the add instruction, I want to know how to modify the instruction of sub)

I have tried following along this guide:

How to add a new custom instruction in the RISCV instruction set

but this is for creating a new instruction not creating a custom one.