I believe that if you dig into the 411-d4 regs that contain the ESOP exceptions to the anti cutback rules you will find and "example 3" that allows a stock bonus plan to distribute in cash in such an event. Practitioners that follow this exception do so by taking the position that by definition an ESOP is a stock bonus plan that contains and complies with those IRC provisions listed in IRC 4975. Others who have not read the regs (my only guess as to why they take this other position) believe that the ESOP exception in IRC 409 that allows ESOPs to "modify" their forms of distribution in a nondiscriminatory manner extends beyond those regs. I dont buy that.
Amending into a PSP still requires relying on this approach. The PSP restatement is to avoid the "primarily invested" standard for sn ESOP post transaction