Will I loose my iCloud email name I change the Apple ID on my iMac m1

You will not lose anything. You can simply "sign out" of the Apple ID you do not wish to use, and "sign in" with the one you want to use.

Will you lose your existing iCloud email name? No.

Having 2 different Apple ID and trying to use the new feature to use an iPad with the iMac does not work with 2 different Apple ID .

No, it won't. You can have as many Apple IDs as you wish, but having more than one will just complicate your life. One person, one Apple ID. Each Apple ID will retain its own purchase history, email address, etc.

If your Mac's Desktop is cluttered with too many files, you can Edit > Select All, then File > New Folder with Selection. That way, all those files will all be consolidated into one folder and you can decide what to do with them later.

When I switch from one user to the other I can not get into the others files or apps. some how I am sharing storage space in iCloud but can not access another user ID unless I sign out and sign into that ID.

That is correct. One Apple ID is as separate and distinct from another Apple ID as they are from anyone else's, notwithstanding the fact they are both yours. They cannot be merged nor can one Apple ID access the contents of another Apple ID.

The sole exception to this rule involves Family Sharing: Set up Family Sharing - Apple Support. And even in that case, Make sure that each family member has a unique Apple ID - Apple Support.

iCloud storage is separate and unique to each Apple ID. If you elected to Add your Desktop and Documents files to iCloud Drive, then your Mac's Desktop and Documents folders will be populated from the Apple ID you used when setting up the new Mac. That may explain what you experienced.

So to be clear...

It is ok to change the Apple ID of the device and will not loose that email or the files and apps with that ID?

Yes. Sign out of one Apple ID, and then sign in to another Apple ID. Nothing will be lost. Each Apple ID account will retain its own files, apps, documents, email messages... etc.

will it combine the 2 Apple ID files or will I have to sign on as another USER with that ID?

The latter. Apple ID content cannot be combined. It can only be shared, as Apple explains in their Family Sharing document.

Thank you John,

what if I do not want all those files on all my new devices. I did not want that same problem to happen with my old screen being my new screen.

Also will I loose all the files that belong to one Apple ID if I change to another name.?

I have 2 user ID on my Mack air. When I switch from one user to the other I can not get into the others files or apps. some how I am sharing storage space in iCloud but can not access another user ID unless I sign out and sign into that ID.

So to be clear...

It is ok to change the Apple ID of the device and will not loose that email or the files and apps with that ID?

How will the device stole the files from that ID? will it combine the 2 Apple ID files or will I have to sign on as another USER with that ID?