Yes, I see how you came to this idea. But I can say that adding say 4 new pieces with new movement and keeping the rules the same is merely a tweak as well. I think it is the net result that is important.
True you could change the wording a bit, take out a few lines of code in zillions but its still a major change. I don't see how this is any different from replacing say a queen with an amazon. or a rook with a chancellor.
So, I would say this is a full fledged chess variant, but I do see how you see it as a 'tweak' to the rules.
Just that this little 'tweak' changes things drastically - you have to admit!
An interesting idea nonetheless ...