My home was build in 1920 and uses planks for the roof deck. I was planning on just removing the old asphalt shingles and installing new asphalt shingles. Makes sense to me; until a contractor friend told me it was code to install OSB or plywood over the old 8 inch planks. I don't see why this would be necessary if the planks are still in good shape?
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.