Commonwealth Bank has forecast the impact of potential changes to the capital gains tax (CGT) discount and negative gearing could fall flat for prospective home buyers, with only a "modest" decrease to house prices.In a pre-federal budget analysis, economists from the big four bank, Luke Yeaman and Harry Ottley, said reforming the property tax benefits would only have a modest impact on lowering house prices and increasing productivity.It estimates house prices could fall by 1.4 per cent with changes to