Should consolidation autologous stem cell transplant be considered for de novo follicular lymphoma transformation to DLBCL after achieving a PR with R-CHOP x 6 cycles, if residual disease showed original indolent follicular lymphoma?  

First, it is important to make sure that residual disease is FL rather than tFL, which it sounds like you have already done.  However, the PET scan should be reviewed; if there is a different node that has very high FDG uptake, then that node should be sampled to rule out residual DLBCL.  ...