B&M GP40-2 #311's Truck Swap

I remember as a kid seeing 311 with leaf spring Blomberg trucks. I know that the 300s were rough riders first hand. I was assigned an ex B&M GP40-2 last week on my D&H assignment. Was the swap due to an experiment on the ride? Maybe a spare set was used for use during truck overhauls, or even due to a wreck, possibly? I remember the brake rigging was converted to single shoe.

It originally had the newer Blomberg "M" type trucks... best I can tell from online photos is they were swapped out sometime after 1983 but before 1985. 311 is (as recently as early 2014) still around, as Ontario Northland 2200, and it still has leaf spring trucks.