To update your weapons file in an old scene so the newer FPS will run, then go to the "Main menu > Editors" and open the weapons editor.
Click the open button on the top middle area on the weapons editor to open a dialog box.
Navigate to the default scene folder in "$(SkylineSDK)/Skyline/Data/Scenes/".
The dialog should show 2 weapons files. Choose or double click the "Default.xSkyWeapons" file in that folder and then save your older scene. Now place the new FPS in the level and it should work as will the older FPS weapon.
This is sort of a porting note that everyone can follow