Locate the user database file on your computer at: C:\Users\[USERNAME]\AppData\Local\pix4d\common\16\ucmdb.xml where [USERNAME] is your Windows computer login name. The AppData folder is hidden so you will need to enable hidden files and folders in your file browser in order to see it.
Open the xml file in a text editor like Notepad (should be able to just double-click to open, otherwise open your text editor and drag the file into the open editor window)
Download the camera profile above that corresponds to your modified camera.
Open the xml camera profile in your text editor
Copy the text from the <camera> to the </camera>
On the ucmdb.xml file that is open in your editor, paste the copied text after one of the other </camera> lines. If no </camera> lines exist, copy it after the <vendorId value="pix4d"/> line and before the </cameraModelDB> line. Example xml download HERE.
Save the ucmdb.xml file
Relaunch Pix4D and either start a new project or open an existing one.
Click the Camera icon (Image Properties Editor) at the top left of the main window. If this is a new project, go to the next step.
Click the "View..." button under the "Selected Camera Model" section.
Click the drop-down from the "Camera Model Name" and select the profile you have added, which should have a "user" icon (upper half of a person profile).
Select the "OK" button at the bottom and run your project.