Can these be said:
I'm changing the bracelet on my watch to a strap.
I'm changing my watch bracelet to a strap.
I'm gonna change the bracelet on my watch.
I'm gonna change the bracelet to a strap on my watch.
The strap can sometimes be called a bracelet. Some watch brands call it a bracelet because it's made of thick metal.
In that case all of them are right.
The third and fourth ones are wrong because of the use of "gonna". Which, as you are well aware, Batman, is not an English word.
In casual speech or emails and texts to your friends, Batman, you can use 'gonna' to your heart's content.
Here on the forum we expect you to use standard English to set a good example to other students.