I choosed Tribal LW before TBC came out and was currently resto at that time.
Now I've changed to feral spec. Is it possible for me to change to Elemental LW? (without unlearning the whole profession and loosing my 366 LW skill) =/

You can switch without dropping leatherworking. I was elemental before BC came out, because I was a hunter but my guild already had a dragonscale LW. After BC came out I had to switch to dragonscale. I don't remember the specifics, but I think you would go to your original trainer for tribal, and there you can say you don't want to be tribal anymore. Then you find a book in Tanaris I think, which lets you pick your new specialty. You also have to pay, something like 40-50 gold. And you do not have to do the quest for your new specialty.

If you look around in previous pages of this leatherworking forum, you should be able to find more specific information.

Just posted something similar in another thread so I don't mean to intentionally repeat myself.  Yes, you can switch from Tribal LW to Elemental LW or the other way around.  BUT you can ONLY DO IT ONE TIME.  For example, here is what you should NOT try to do.

Gain Tribal Leatherworking.  Decide it isn't for you.  Switch to Elemental working.  Decide you hate it also.  Attempt to switch to Dragonscale Leatherworking thinking "This is the one for me."

The trainers will let you know during the first switch that you're about to do something which cannot be changed and that you need to be 100% sure about the decision you're making.  I strongly suggest doing a little research on the patterns of each spec before choosing one.  I was originally Elemental leatherworking but as an elemental spec shaman this was useless and I switched to Dragonscale.