I am not too sure as it has never come up (we don't use multi-tap CTs). I was interested myself in knowing the answer but a quick search of the internet produced no clear answer.

If the CT is rated at 15VA burden then it is probably 15VA when connected 1000:1. 500:1 will result in lower secondary winding resistance and therefore allow a theoretically higher burden, but I don't expect the turns ratio to burden relationship to be linear (ie- 500:1 ratio to allow a burden of 30VA).

http://en.wikipedia.org/wiki/Current_transformer

For safety reasons I would assume that 15VA is your burden limit even if you intend to use the lower turns ratio (500:1).

Alternatively you could test the CT or discuss directly with the CT manufacturer.

Yes! The burden will remain same as long as the secondary current is same.