It might be possible to use the small "post it" self adhesive notes as a masking material. They are very low tack and hopefully will not adhere too well on the upper wing crosses. You can lay them on top of the crosses at a right angle to each other to act as an improvised mask.
If by chance they do cause damage to the upper wing cross decals, the upper crosses should be fairly easy to replace, or possibly even spray back on if they become damaged. I have also seen various pictures where the real life 1 to 1 scale crosses had overspray on them as if they were not masked off at all during the repaint in the field.
Look at the fuselage cross on this Fw-190. Part of it is actually appears to be missing ! It was a repaint in the field... as evidenced by the over spray on the cross at the 10 O' clock position, just next to the vertical bar behind the chevron. The center part of the cross was sprayed over using a darker camouflage color. Typically this would have been a black color, but you can tell that it is not since it looks to be a different color from the chevron and vertical bar... which are presumably black.
This could be an option too. Try to free hand spray the new colors on and paint as close to the edge of the cross as you can. If a little over spray gets on them so be it. Who can prove otherwise unless they come up with a picture ?
It's yours so re-do it in the manner that makes you the happiest. That's all that matters in the end.
Please keep us posted, on the use of the post it notes (and I hope this helps)
Thanks 🙂