Don't get rid of your toilet downstairs, it is so important for so many reasons:
a) kids - I've had several incidents when they don't quite make it up the stairs in time! - oops
b) guests - I have to completely clear out my bathroom every time guests come because the family bathroom is also the guest bathroom and I don't want everyone seeing our wet towels and guzillion bottles and tubes of cosmetics, etc. It is a real pain.
c) Old people - my mother and parents-in-law come over a lot and it is difficult for them to go up and down the stairs.
d) Sickness - if you are ever ill or can't get around easily, it's much easier to have a loo downstairs so you don't have to keep going up the stairs.
e) I believe houses with toilets downstairs are more desirable to buy - although I don't think this is a deal breaker as anyone who bought it from you could just put one in.
I think if you are going through all the disruption of building, and you want a bigger kitchen, then reconfigure the toilet and have a smaller one rather than getting rid of it. You may regret it in future and then it's a hassle to put it back in.
I am planning to do a side return in my house some time this century and when I do, the first thing I'm putting in is a downstairs toilet!!
But of course it is your home and your choice! Good luck!