Hi,
Well, the MyParcel plugin is for the MyParcel shipping system.
So it would require developing a Royal Mail plugin for the Royal Mail shipping system.
It was just to give an understanding of what such plugin could do.
You could modify the MyParcel plugin for the Royal Mail plugin, yes. That would basically mean rewriting the whole plugin besides a few lines of code for the structure of the plugin. It's like payment plugins: they are all based on the same structure. So you can use the PayPal payment plugin to write the Stripe plugin, but if you look at their code, besides a few lines of code which the same because they use the same HikaShop triggers, same HikaShop variables/functions, most of the code is actually different.