With practice it can be done, but on current pens it is not advisable. Nib and feed are a tight fit in the nib housing; pulling them out carelessly bends the nib, damages the feed or cracks the section. The damage then quickly exceeds the sum you were hoping to save.
Montblanc does not release individual replacement nibs to the open trade; they can only be had through the manufacturer or an authorised dealer, with professional fitting included in the price. For a gold nib for the 146 you had to reckon with something like 250 euros as at 2010 — about half what a replacement nib for a Waterman Sérénité cost at the time.
It rarely makes economic sense. For that money you can often find a well-preserved 146 on the second-hand market in the nib width you want, so selling the pen you have and buying a suitable one in its place comes out roughly even. M is the standard width; very broad or oblique nibs are harder to sell on.