zip roof system or plywood on roof boards?

I have an 50 yr old roof, newer shingles, with true 2" x 4" cedar rafters. The rafters are covered with 1" x 12" boards. I was thinking of putting 3/8" plywood or zip roof system on top. The rafters are in pretty good shape. We live in Toronto, Canada and get some snow. Should I put the plywood on top? Forget about the plywood? Remove the roofing boards first?

Thanks!

I’m planning on using Delta MaXX Titan as underlayment.