I've done it on a few a/c. Here's what I remember.
1. Backup the original panel.cfg.
2. Find the panel/subpanel with the gauge entry for the hud. Sometimes it can be a single popup panel, or part of the main panel for 2D and VC.
3. Swap out the gauge entry with the replacement hud gauge. Its best to know where the new hud is located - either in your gauge folder or cab file, and if that cab file is in your gauge folder or in another aircraft panel folder. Just make sure the gauge is loaded when you load the aircraft.
4. If there are any hud switches with the old hud, you may want to remove these with // remarks.
5. Save the panel.cfg and take it for a test flight.
From this point forward I've used the FS Panel Studio for the finer adjustments. With some hud gauges it was difficult to read the numbers unless I stretched and recentered the hud.
As I write this I have my IRIS F-15 panel open in FS Studio. The first panel it shows is the 2D where the hud is clearly visible in the panel and can be easily stretched, centered, etc.
Off to the right side of the menu bar is a tool to "Select a Window." Here you can move to the VCCockpit window with the hud for the VC. It does not display the background bitmap, so its trial and error for sizing and positioning the gauge for the VC.
Hope that helps.
--WH
