Previously I faced the same issue where I was not able to select the project node as it was not available in the drop down to select and my project was attached to sandbox.
You can raise a support ticket to disconnect your project from sandbox and this will enable you to select that project and connect it to your desired node as that project will be available in drop down.
Note: I am not sure about backup of database in sandbox and the above solution will clear your database as well. Please ask this in your ticket that you want a backup of your sandbox database if it is possible.
We have just published a documentation regarding the subject: https://world.mendix.com/display/mxsupport/How+to+connect+a+different+project+to+a+node
Why do you not just create a local mpk and upload that to the regular slot? Same can be done with the database. You can then do the detach attach process later.