When re-roofing and doing a tear-off do I have replace the roof deck with plywood?
Are your 8-inch planks laid tightly together, or is there significant space in between? Old roofs that were originally topped with rigid shingles (wood or tile) were sometimes installed with spaced decking planks, and that kind of decking is not suitable for use under modern asphalt shingles, which are flexible.

However, if the planks are relatively tight together this is not a concern. The planks should be fine as long as they aren't damaged, and you'd only bother replacing damaged sections. Strip old shingles, add new underlayments, add new shingles.