I generally use varnish instead of polyurethane but I think the same principles apply. I don't find it necessary to take the finish down to the bare wood unless it is really bad. Any areas in which the wood has greyed or otherwise discolored of course have to be taken down to clean wood, and any peeling or raised finish needs to go, but areas in which the finish is still intact I would just lightly sand with something like 120 grit paper to slightly rough the surface before reapplying polyurethane.
I haven't tried it, but I don't think I would apply oil to wood partially finished with polyurethane. I doubt that it would hurt anything but I am doubtful it would look good.
If you really want to go from a bright polyurethane finish to an oil finish I would probably go ahead and take the gunwales off. It is a lot easier to sand down to bare wood without scratching up the hull that way and you can seal the hidden wood of the inwales and outwales at the same time. You have to be careful not to accidentally break those long, narrow rails when they are off the boat, though.
I haven't generally had trouble getting wood gunwale screws out except on boats such as Bells in which a couple of long screws near the stems go all the way through the outwale, the inwale, and into the deck plate. As I remember, the deck plates on the Rapidfire sit atop the inwales so you shouldn't have that problem.
If I remove the gunwales I now generally sand the inside of the outwales and the outside of the inwales and apply epoxy as a wood sealer. I think it provides better protection from wetness and holds up better than anything else. If you don't have any I would use polyurethane or varnish on that surface even if you are oiling the part that shows as I think it is more durable.
PFD's (Life Jackets)
Shirts / Tops
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