Post some pictures of the current set-up from the side, so we can see what the top of the pier is like and the current adapter plate under the mount. If there is an owl hole, try and get some pictures inside there too looking at any bolts.
If it was me, I’d remove the old adapter plate, clean off the top of the pier, maybe re-paint it, and then attach the new adapter plate. If you know what sort of pier it is we could work out if the new plate would fit. Do you have any old photos of the pier with the current plate attached to see? Or whoever you purchased it from may know what adapter plate the EQ6 one is (where he got it from) or have some photos.
I think if you can do as much planning and preparation in advance of taking the whole kit down, your kit will be out of action for the shortest time possible.