Not by Montblanc. The manufacturer keeps neither spare parts nor nibs for pens outside the Meisterstück line that have been out of production for more than ten years. A factory nib change for a 342 — a slightly superior school pen of the third series — is therefore not to be had.
The usual route is through collectors and restorers who keep used original nibs in stock. A suitable nib for the 342 cost between about 50 and 100 euros in the mid-2000s. If you trade in the nib you have, experience suggests around 25 to 33 per cent will be allowed against the price, and rather less for unpopular widths. That applies particularly to an OM nib, an oblique medium, which is seldom sought after and correspondingly hard to sell on.
For identifying and placing Montblanc pens made between 1946 and 1979, “Collectible Stars” by Jens Rösler and Stefan Wallrafen is regarded as the standard work; most models of those years are described and illustrated in it. It also answers the question of which models were built in a given period, which helps if you are looking for a pen from a particular year.