standard behavior is that attribute, which has been created by relationship(by migrating form parent entity) will be delete, when relationship is delete.
Is there way how to change this by Macro. Each attribute has internal property "KeepForeignKey". When this property is true, attribute preserve after delete relationship.
I wrote simple macro for you, it set "KeepForeignKey" for selected attributes. Package is created for version Toad Data Modeler 6.1.
Please copy this package to location "Path to Packages" (To show this tab in settings you need switch on Expert mode from Options-General) and start TDM. (You can use function "Import Custom Package" from Main Menu - Expert Mode - Customization).
Now when you right click on attribute in Model Explorer you will see new popup item "Keep Attribute"
Code of Macro is very simple, you can see it. If you want you can create easy macro to modify all attributes in model.
KeepForeignKeyMacro.zip (1.14 KB)