I went through something similar with mine - I installed riser bars from a dew.

I had some problems with hand numbness, but then replaced the grips with ergo grips and haven't had any problems since. My commute is 7km one way, and I've had it out for a few 2.5h rides now with no more hand problems, and the bike is very comfortable.

If I was doing it again, I would probably replace it with a flat bar with bar ends or something like trekking bars so that I could have a more stretched out position every now and again.