Configure classic workspace for AutoCAD 2015

Solution 1:
Open the Customize User Interface dialog box (or type the command CUI).
Select the desired workspace or create a new one.
Use the plus symbol to open the Toolbars tree in the left window.
Drag the toolbars you want to the toolbars tree in the right window.
Click Apply and then click OK to close the configuration editor.

Solution 2:
The classic workspace can be imported from a previous version:
In the Ribbon - Manage Tab, click "Import" (or type command CUIIMPORT).
In the customization editor, click the Transfer Tab. The current configuraton file is in the right pane.
In the left pane, load the configuration file (acad.cuix) from AutoCAD 2014 or an earlier version
(C:\Program Files\Autodesk\AutoCAD 201X\UserDataCache\en-us\Support).
Under the workspace list in the left pane, drag and drop "AutoCAD Classic" to the workspace list in the right pane.
Click Apply and then click OK to close the configuration editor.

Source: http://knowledge.autodesk.com