Q: can't change my apple id

Yes, I have this very page up on an adjacent window as I typed my question. The support pages seem to be getting better. But, no help from this page. I am logged out from the apple store, and my id account I want to get rid of. I can't think of any other service I would be logged into. It's a pretty plain vanilla setup, I just want to get rid of this 10+ year old id. I want to slim down to one id across the entire setup. Not happening. Just checked the app store, and I'm definitely logged out.

Well... thanks! This post of yours is actually very helpful. I have scoured the Apple support pages looking for info, but there has been none as clear as this post. Thank you very much. I have long felt that I wished there was a memo with every new product saying, think carefully about what you are about to do and here's why: a, b, c. Whatever. Your choices will hang like a millstone around your neck if you're not careful. It is really crucial to plan this out, but the OS updates and the tech has evolved faster than what I anyway, can manage in a backwards and forwards compatible fashion. I'm stuck. I guess the only way out is to buy all new equipment. Hey! Ya think? it could be that simple?

Thanks for your help, it is very clear.

Yes, "giving it to myself",  that is a good way of looking at it. Kind of the nuclear option! I followed this link to a couple more pages. I was interested to see that for El Cap, one of the back-up options is iCloud, where "you can back up your music" before migrating to a new machine. Now, one thing I don't have is a lot of purchased music, maybe less than 1% of my iTunes library, and none of those songs are critical files. I wouldn't lose any sleep over them if they vaporized. But I know a lot of people have libraries of mostly all purchased music, and they may have figured out how to sync those files around to their phones, pads, whatever. But those files were purchased using a particular apple id, which, as ckuan says " All content are forever tied to the Apple ID used to obtain them", so that means that forever more, wherever those people go, those files are going to require that the apple id used to purchase them be used. So those folks are maybe someday going to have to choose between keeping apple ids they may not want or need anymore, or re-purchase the content. Do I have that concept correct? I think I do..

This brings up a question: do "files" like the contacts or calendars I created, become protected digital content like an app I might have purchased from the App Store? Could I actually migrate my Contacts file to a new computer with a new apple id, or does that Contacts file have some tag attached to it that requires the old apple id? Like, if I had unintentionally stored that data in iCloud? I think, I hope,  the answer must be "no", there must be some difference between personal info and purchased content. But, does using iCloud, an Apple service, tag the file as some kind of proprietary Apple DRM? I have never found any Apple support page that addresses this or related issues, that of the the permanent DRM lines of constraint that build around a user when they establish a digital identity. You know, some paper that clearly addresses the issues of  "these are the things you should think about, and this is what's going to be the consequences of your choices". On the contrary, they just have blithe info like, save your DRM content on iCloud! These apple ids are like tattoos, but much more difficult to remove!

Thanks for your help, it is much appreciated!

ckuan, I'm not following you about the OS being downloaded or pre-installed. This particular machine is a "late 2014" Retina 5K iMac. I forget which version of OSX 10.x.x was pre-installed on it. But I have been installing each upgrade as it came along and now it is 10.11.4

So, it seems to me I have a combination of pre-installed and downloaded. Now what? Up until now, I have been manually transferring files around my computers and it seems to work OK. Exporting the Contacts and Calendars files to a hard drive, then transferring to my MBP when I would have to go on a trip. Works, but not very elegant. I think that if I go forward with this nulclear option, I will be saving my user-created files on disk and outboard drives before I do any wiping.

What does OP stand for?