Off course first movie belongs to Amitji, Not Don the character..!!
What was in first Don actually, how much was Don in it, Amitji spent more then half of movie as Vijay, Don got killed, Vijay's story was followed and end..!!
In SRK's version, it was all Don, from start to end, story belonged to Don, movie belonged to Don, movie started with Don, movie ended with Don, SRK & Farhan Akhtar gave Don an identity which original movie didn't..!!
Amitji's original was just a movie whereas later 2 movies were story of Don the character.
Amitji's Don can be renamed as anything else and it wouldn't make a difference, SRK's Don is movie about Don, it's it's identity, you can't change that..!!