That might be alright if you only use your bike for crits and/or you live in a flat area. Why not just get a new 12-21 cassette and leave the crank as it is, then you have the option to readjust the end stops if you are you are going to do a ride or race where there is going to be some climbing. Since you are apparently on a budget, there's might not be much point in sinking money you could use on race entry fees into converting your bike into a double.