Its a part of my designs also to be able to move the spindle up to clear for a large work piece. But in real life that would mean realigning vertically the spindle, and its time consuming process. The only proper way to do this without fiddling is to make a custom mount and machine the spindle so they fit perfectly, as i make now on my machine now. But its expensive.

having in mind at which stage are you and the prices of ball screws and rails for Z, if simply you made the error and bought them short, spend now 100 quid and buy new ball screw and rails, instead of compromising the whole investment. If don't have them, all the better. The Z should be made properly, not to speak that you want a high performance Z.