What happens to a mortgage upon subdividing?

[Pearce G.]

My question is in the subject heading, but here's the hypothetical:

Let's say I buy 5 acres for $500,000 using conventional mortgage financing.  There's one house on the property.  I decide to live in the house with 1 acre and subdivide the rest into 4 additional 1-acre parcels that I will sell for $50,000 each.  Do I have to re-finance them as individual parcels before I can sell them?  Or do I re-finance at closing each time a parcel sells?  Or do I just keep the original mortgage until it is paid off, or the last parcel sells?  

It seems like I would have to get my lender's approval of each sale, since the value of the underlying security would change each time a parcel is sold.