Well, I've not done this on a ukulele specifically, but I have had a degree of success. It all can depend on your definition of "decal". I always recognise this term as an alternative for "water transfer", where the image is supplied on a sheet of paper, supported by water-soluble adhesive. The whole is immersed in water for as long as it takes for the adhesive to loosen, then the image is slid off the paper into it's final position and allowed to dry. This type of "decal" I've had no success with, but did hear of a professional photo-retoucher repairing one on a motorcycle tank.
However, if, by the term "decal", you are referring to the type of "sticker" that is peeled off it's original carrying backing then positioned in it's new place by virtue of it being self-adhesive, these can often be "floated off" using WD40 lubricant to soften the adhesive, then allowing everything to dry once the decal/sticker is in the required position.
Please do note :
I have no way of knowing that this procedure will work in your situation. Also there may be a risk of staining the wood with the lubricant if it's not varnished/laquered under the sticker/decal.
I use the very specific brand-name WD40 simply because I know this procedure can work with this product. Other products may be suitable (or not).
Some warmth (low heat) applied to the decal/sticker may be helpful ... again, you'll have to be the judge.
I'll wish you luck, with some confidence, and sincerely hope nothing goes wrong, but I can take no responsibility for any problems resulting.
