Krusty is right... its a bit of a PITA... but I think it's worth it. Especially for squad ops...
In another thread, Krusty lays out how its done... and K man, if you don't mind I'm going to just post what u said here.
This is what I did pretty much verbatium... My only additional tip (and he may have mentioned it) is to write down what skin folder you got the skin file from.
<> Good luck.
"In HTC there is Aces High II, inside that is "cache", inside that are many things. Take note of "stdshape". Open a window to this folder.
Open a NEW folder of "cache" (one step above the first window).
Let's change the P51B. Scroll down inside the "cache" directory until you hit the first "p51b" folder. It is called "p51b1" There are sequential directories named p51b1 through p51b6. Open one.
NOW here's the tricky part. You don't know WHICH directory contains WHICH skin. So once you copy some files you have to open an AH2 film with a P51B in it, check the skin, and try again until you get it right. This is what I forgot to write down. I forgot to note WHICH p51b_ I liked best. I love the one with the RAF camo on top and the partial invasion stripes underneath the rear fuselage but with OD upper wings. Superb skin!
Anyways, so in "p51b1" you have:
dude1
dude2
markings
p51bart2
p51bsid
p1btop
I really don't care about changing pilots, so forget dude1 and 2.
On the p51 you HAVE to include "markings" and "art" because if you don't you'll have screwy markings showing up. Some use new markings and art files to erase the default skin's markings/art (they make it transparent, and if you don't replace it it's very visible and very out of place).
So I go to stdshape's window (still open). hit "P" to find the p51____ files. CTRL C, CTRL V. Drag and copy files from p51b1 into stdshape, hit "yes" on "replace fils?". Do the same for "markings"
Then try out the film viewer. It will also show up as your default skin now.
Okay, whoops! That's not the skin I wanted (for example). Okay go into p51b2 and copy those into the stdshape folder. You're replacing non-original files, so don't back them up again.
NOTE: p51b2 has a file that p51b1 did not have. p51bart1. You will have to back THAT file up.
What I did was just made a "backup" folder inside "cache". I'd literally copy all the files for a plane into this folder. Then when I dragged stuff over and tried 5 or 6 replacement skins in a row I knew I'd have all the originals backed up. Alternately you can change video settings and make AH recompile the cache if you want to revert to defaults again."