Constant-pressure molecular dynamics has been used to simulate the six- to four-coordinate downstroke pressure-driven phase transition in B1 nanocrystals. The nanocrystals considered have previously been formed in upstroke B3→B1 simulations, giving them an amorphous surface region and interior Σ3 grain boundaries. Nucleation occurs in the interior of the crystal, with multiple nucleation events observed along grain boundaries, in contrast to previous decompression simulations of single-domain nanocrystals with crystallographically well-defined surfaces. Competing mechanisms give rise to B3, B4, and d-BCT domains in the productstructures. Four distinct mechanisms are observed, including two B4↔B1 mechanisms. The B4↔IB1 mechanism is the same as seen in previous simulations of single-domain nanocrystalline and bulk systems, while the observed B3↔B1 and B4↔IIB1 display a B3↔B1 correspondence, in agreement with experimental observations of repeatedly transformed CdSe nanocrystals; both of which are different to those seen in previous upstroke simulations. The interaction between these competing mechanisms determines the domain structure of the product nanocrystals.