hook error

[MarkHollis]

Hi, first of all I am not a programmer and just work through logically subtracting and adding. After I upgraded I had the message:
Error loading mission
Hook: {FuelTank04} not found.
I did several upgrades so found it tiresome to pin it to one version.
The clue is the description of the fault - fueltank (Plane). When I was loading in the skins for the planes into paint schemes I was merging the two folders when if fact I should have deleted the old folder with the same plane model and put in the new one. Mixing the two seemed in my case at least to mess up the operation. I know that some may say well of course you do that but I thought the new plane upgrades were "Top ups" of the old not replacements. I may be completely wrong and if someone politely points out the real reason they are welcome.