I did the above and it kind of worked. I powered up my old iphone (the one I'm getting rid of), "went to the app store...Scroll to the bottom, and where you will see "Apple ID: "aaaaaa@aaaaa.com". I clicked on that and you have choices "View Apple ID, Sign Out, iForgot, Cancel". I clicked "Sign Out". Now I went back to my account details on our iMac and looked at the list of devices and the greyed-out "remove" was now NOT greyed out any longer."
My old iphone wasn't greyed out, and neither was my ipad and my new iphone. My other laptop was still greyed out though. So ... I don't know why logging out of one triggered the others to show up too, but it did. Worth a try. FYI, fortunately I still had the device I want to remove. But I'm wondering if it would have worked if I had done the same on my ipad, and maybe that would have released three other devices. I don't know.