Montblanc settles most service work on a flat rate that bears no relation to the particular fault. As of 2008 that flat rate stood at 59 euros for a ballpoint with a broken cap and for a propelling pencil with a faulty lead mechanism; in both cases the pen was replaced outright. Changing a nib from B to F came to 39 euros and likewise ended with the 146 in question being swapped for a new one. Changing the nib width is thus perfectly possible, and cheap set against the price of a new pen.
What matters here is the consequence of that practice: instead of repairing a pen, the factory service will often hand back an equivalent new one. For a pen in daily use that is convenient and inexpensive, but for a collector's piece it is a real problem, because everything original disappears with it, the old nib with its tip worn in to the owner's hand, the imprint and the patina of years. For older or rare pens a specialist repairer who preserves the original parts is therefore the better choice; for current models the factory service is the straightforward option. Anyone sending a pen in should establish beforehand whether it will be repaired or exchanged.