upgrading to ultegra wont be cheap, probably around 400 bucks and that isnt for the whole groupset, or you can spend 500 and get a full 105 5800 which is all you would need, if you like the bike then keep it, if you want to upgrade your bike later on, just take off the whole groupset, slap on the old one and sell it.
in my opinion buying the whole 105 5800 groupset is much more economical than just changing to ultegra shifters, they will perform similarly, and the cost wont be that much more since you have to change the cassette, detrailuers etc...