Isn’t it just changing promotion requirements
No, it isn't just as easy as that.
I know how to make promotions for specific units, and I can already see this implementation as a janky one.
If you just change the promotion requirement, you change it for the whole ranged units which not in this scope of the proposal.
Skirmishers borrow the Archer unit combat type, which means that you have to make a new unit combat type, which has its own complication where you must make sure all promotions, buildings, units, and other elements work properly. It's going to be a huge task for a simple promotion implementation. Because unit promotion works on unit combat type level, not unit class type.
I'm not a fan of janky implementation for base VP. If you want to do it properly, you have to do the painstakingly all the steps above.