Asking to present paper as first author, when the second author's travel has already been booked
I think it is certainly appropriate to ask and wonder why you hesitate to do so. I think that a note that you are able to attend and that you would like the opportunity to present the work and that it would be a boost to your future career should be received positively. 

Having registered for the conference (the advisor) should mean little or nothing in this situation. Whether it is good politics to demand your right to do this is a matter of personalities, of course.  

But the fact that you are spending your own money likely won't count for a lot. Your presence at the conference should, however arranged, count in your favor. 

But, yes, it is reasonable to ask, and I think it would be unreasonable for the advisor(s) to refuse you. 



I'll also note that there are a lot of reasons to go to CS conferences beyond presenting something. Your advisors should recognize this. They haven't "wasted" money if you present in their stead. There are normally a lot of ideas floating around and a lot of interesting people to interact with.