Steps to reproduce:
1. Download and open attached project.
2. Open scene named "main".
3. Select game object named "GameObject" and notice that it contains "Script" field which indicates that the currently attached script is named "test1".
4. Navigate to the "Project" window, select script named "test2", drag and drop it to that "Script" field.

Actual result:
The color of the field is grey. So, the scripts cannot be swapped. This method is not very efficient, since now the user has to remove the old script and add a new one separately.

Expected result:
It should be possible to swap the scripts. This functionality works well with older versions of Unity and some users found it very useful.

Reproduced with:
5.3.1f1, 5.3.4f1, 5.3.5f1, 5.3.5p1, 5.4.0b18.

Cannot reproduce with:
5.1.0f3, 5.2.4f1.

Note:
GIF attached.