Sorry I couldn't reply sooner Susan. I am working a temporary job until the first of the year and have a crazy, crazy schedule.
If you have a garden, they make good fertilizer I am told however a certain biologist had an over abundance of very large squid carcasses to dispose of and the neighbors complained of the smell
(not a problem with a single animal of this size.
Depending upon how many I have in the house, I preserve the octopuses that die in good condition. Over the years they have gone to various students, either for projects or plain curiosity. When I send them the only requirement is for a photo of their final use that I post at the end of the journal. @gjbarord
mentioned wanting cephs for his classroom and I offered the group I have "in stock" (not many requests in the last couple of years) but I have not heard back from him so he may have all the octopuses he needs.
If the new LR is in your existing tank then it is already cycled. You will disturb the 30 somewhat and should probably monitor closely and do a couple of extra water changes after removal if you see any change (the substrate under the removed rock may go through a quick cycle) but generally speaking you can move a rock or two from tank to tank without any impact. If you move the rock now, you will not have any problem when a new animal arrives.