This is no hoax, if you want a video I'll gladly record one. Heck, here are the codes to switch your sprites with him yourself.
--VisualBoy Advance Codes--
If you have VisualBoy Advance you can put these in by going to Cheats > Cheat list... and clicking on the "Code..." button. You can paste them all in at the same time. It only works with The Lost Age, I don't believe his sprites are in the original Golden Sun.
03002100:DC
03002107:03
03002108:0830E920
0300210C:08304FE4
I've noticed some graphical glitches as a result of these codes, but no major problems. They don't change your sprites on the world map, in battles, or on the status screen. Also, after entering a new area you may briefly change into a different character.
--Action Replay V3 Codes--
It's a lot longer than the VBA code because it also changes your sprites on the world map, and is adjusted to prevent a lot of the graphical glitches it otherwise causes. The glitch where you change into a different character after entering a new area is still present though since it's directly related to changing your sprites.
Golden Sun TLA Master Code
D06E067F 8055B26F
E0CE785B D8990E3E
Play as Link
759BE646 09FCAF90
C8F712A9 DD7B08F2
DD9FA7CB 542FB102
AF9C69C9 54A98243
AB223216 631A0C4F
1E27D83B BBFB6923
2D190863 3F2DC15A
E768D406 4E9779DC
7C639D9F 4C442A5D
C6A11B07 D4D76256
ADB85D93 6D4F1230
1E27D83B BBFB6923