is it possible to refine income tax?

Income tax seems to be too monolithic, either it is set high or low, but tax systems are never that two dimentional. For example would it be possibly to introduce tax bands? Say setting a progressive or regressive income tax? In most states people that earn more money pay higher income tax rates, so is it possible to introduce a system where it is possible to set the basic rate of tax, and then introduce higher rates for high earners? Even introduce a 0% rate for very low earners. Likewise some countries have introduced a flat tax. Different income tax rates have differnent effects on different social groups. Think of Gordon Brown’s recent abolition of the 10% rate of income tax in the UK, that was very poorly received by the electorate because it was seen as regressive. In the game income tax is progressive and sales tax is regressive, but in the real world this does not have to be the case, income tax is regressive in places with a flat tax. Would it be possible to introduce such a change?

not easily by modding, at least not with it still being a single policy. You could replace it with 3 different policies representing 3 different tax bands, each with differing effects. That should work fine.