How far can genetic engineering influence those already living?

The DNA in every dividing cell in your body is subject to change, but I don't think that's what you mean.

Many cells in your body are differentiated- they have reached a terminal state, are executing a set genetic program and no longer reproduce (muscles, nerves, epithelial cells, endothelial cells). Other cells are undifferentiated- basal cells, stem cells in the marrow, etc. Those could be targeted, leading to new differentiated cells (epithelial cells come from basal cells, blood comes from the marrow, etc) with transgenes.

 

There are lots of political forces here at work in what is 'moral' and 'immoral' in genetic engineering.