you have a very unique problem
The problem is not unique. In fact everyone who changes his nick has to face it.
eg. Fuzzy had the same problem in May 2002, currently Puter n me have this.
There is some old bug in the code... ...isn't it.
You can upload your skins with your last account. It will always belong to you. I have the same problem still when I upload it goes into my same old gallery. So you should have no problem uploading. Just try it once.
When your problem is fixed, I suppose your both accounts would be merged.