Basically what you want to do is enter the Skylander Management Screen (- button on Wii) and scroll over to "Mange." Now, scroll down to "Ownership" and select "Yes" you want to take ownership of this Skylander.
You do not loose any progress when you do this. All of your Heroics, money, upgrades, and level progression are fully transferred.
Those wanting to do this are likely victims of the Weapon Master Glitch. There is no point in playing new characters and what not on a save file that you will never get a full completion on because of the inability to complete Chapter 21-22, so why not move the Skylanders and work toward your Accolade goals on a new save file? That was my logic, and I am thankful we don't have to start from scratch completely.
This will also be helpful if you say traded your spare Stealth Elf for a friend's Voodood etc. You can both take full ownership and have access to the Heroic Challenges as if the character was your very own from the beginning. I think this is pretty simple and between this post and the video I do not foresee anyone having too much trouble. If you do have questions, feel free to ask.
Here it is in laymen's terms:
How to Transfer Skylanders Without Loosing Any Data
1. Open the save file you wish to transfer Skylanders to
2. Place the Skylander you wish to transfer on the Portal of Power
3. Open the Skylander's Screen (-button on Wii)
4. Scroll over to the "Management" tab
5. Scroll down to the "Ownership" button
6. Click "Yes" I want to take ownership of this Skylander
7. Repeat as needed
-You DO NOT LOOSE progress (level 1-10 etc), Heroic Challenge Stats, Upgrades, or Cash. This is safe and will save you from having to start from scratch.