Re wire my speakers ?

I have a pair of JBL L150A which I don't plan to change in the near future.
My speaker cables are Audioquest Granite.

Well, I was thinking if changing the internal wiring would have a positive impact on the sound ?

If you're pleased w/the sound, leave alone. Whether changing
the internal wire will have a positive effect depends on your expectations. If the wire is anything like the wire inside my former JBL L40 you will be shocked to see how small guage wire was used! Who's to say this is good or bad?
Can improvements(changes)be made, I'd say yes. You might consider changing your spk. cables instead.
If you are pleased with the sound why not make them better. Improving internal cable can make a lot of difference. Many companies really skimp on the wiring. The nice thing about this is how easy it is to upgrade the cable.

I bought a pair of speakers at a point when I was on a very tight budget based on the recommmendation of another audiophile. I set the speakers up, turned on some music, and almost cried. They sounded thin, lifeless, and tinny!

After several days of listening, and the knowledge that they weren't going to get better I got out some wire I had (I can't remember if it was monster or audioquest) and rewired the whole cabinet (binding post to crossover, and crossover to driver). The difference was immediate and obvious. Rather than getting rid of these speakers right away I ended up keeping them for a while. Friends were surprised how good they sounded based on how they looked.

The speakers were a pair of Wharfdale Diamonds, and yes they did sound good...eventually!
Hey, the odd tweak can't hurt. It is only wire, and you can go back to what it was if you want. Just be careful. Save the cabernet/Jack Daniels, or whatever for after you put the skrew driver away. Well, I do somewhat better after the first glass. Just not so well after the....'second'.....
I rewired my 6.9 norh speaker,with siltech wire,the
difference, night and day, I think if I will blind
fold Michael Barnes the norh founder,Ill bet He
cant easily guess it. Tweaking helps, if you know
what you are doing.Thats why I send my sony9000es
to modwright, My fast experience tells me,It works.
I rewired my speakers with the same wire used in my speaker cables. I liked the difference. See if your local AQ dealer can order the bare wire used in the speaker cables you are using. A dealer here (now sadly defunct) was able to order the AQ Midnight wire in bulk for my speakers.