Reusing the existing panel will require you to fabricate a new blank cover for the panel, which the AHJ may or may not accept.
The only potential problem I see with a big PVC box is the lack of ground continuity if any of the branch circuits are run in AC, MC, or metallic conduit. Grounding wedges would be needed to bond the conduit or cable armor at the box.
I generally don't reuse the existing panel, and use a short length of wireway as a splice box if I can get the old circuits into the new panel.
