The Classic Workspace is a feature of the ACAD.cuix, not the ACA.cuix. If you use ACA as AutoCAD, then you could use the ACAD.cuix as your main file. You would lose the ACA-specific contextual ribbons and other features, however.
You could load the ACAD.cuix as a partial cuix file, but that can cause hang ups as both the ACA and ACAD have contextual ribbon tabs for the same action (editing MTEXT, for example), and both try to load. You could have separate profiles for ACAD and ACA, if switching between profiles is acceptable. If you really need both at the same time, I would recommend creating a custom CUIX file and transferring the necessary features from the ACAD.cuix file to build the equivalent of the Classic Workspace. (Never having done that, I cannot say how much work that would be. It may be easier to start with a renamed copy of the ACAD.cuix and delete the contextual ribbon tabs that already exist in the ACA.cuix.)
My intention was to change the architecture version permanently to the regular autocad, and not working on one and occasionally on the other.
From your answer I didn't understand exactly how to make the change itself(which commends to write).
In the Options dialog, on the Profiles tab, do you have a profile called "AutoCAD"? On your desktop, do you have a shortcut called "AutoCAD Architecture 20xx as AutoCAD" [substitute the last two digits of the year of the release of your version for "xx"]? If so, use that shortcut (which will set that profile current) and you should get the ACAD.cuix and be running ACA as if it were out-of-the-box "plain" AutoCAD.
If you have an AutoCAD profile, but do not have an "as AutoCAD" desktop shortcut and want one, you can copy the ACA shortcut you do have. On the copy, right click and choose Properties from the context menu. Rename the shortcut, if you want, on the General tab. On the Shortcut tab, edit the Target string to remove the /ld switch and the string that follows. Change the string after the /p switch to "AutoCAD". For example, here is what my "AutoCAD Architecture 2011 as AutoCAD" shortcut's Target string looks like:
"C:\Program Files\Autodesk\AutoCAD Architecture 2011\acad.exe" /p "AutoCAD"
If you do not have an AutoCAD profile, you should be able to generate one by copying your ACA desktop shortcut, follow the instructions above, making certain that the profile name you use is not on your list of current profiles. (If you do not have an AutoCAD profile, then using "AutoCAD" after the /p switch should work. Use the new desktop icon to start ACA. You should get a warning message that the listed profile could not be found, and that the default profile will be used instead. Unless things have changed in more recent releases, the default profile should create an "as AutoCAD" environment for you. This tip courtesy of Matt Dillon, written when ADT 2007 was the current release: http://modocrmadt.blogspot.com/2006/10/creating-ad
This blog article, written for ACA 2008, has screen shots and a more detailed description of the process of editing the desktop shortcut properties from a copy of the ACA shortcut.
(The article does not explicitly mention deleting the /ld switch and string, but I would recommend doing that as well.)