I have never seen any pictures or events where she has worn this necklace before...maybe in private.
To change stones would not be too much effort....it is the jeweller's job to do so,whether it is time consuming or not....CP Mary is the client. I believe this necklace used on Jan 6, has pearls as well as, other stones in it, in order to give off such a fantastic hue of color. The necklace of 2004 appears quite "flat" in that respect.
The 3 strand necklace in 2004 appeared to be overpowering for her and the gown. Her stature is quite petite. Going to 2 strands puts the necklace more in proportion to her frame. Remember she had only been married for one month at that time and still uncertain of her style and taste and probably lacked a bit of confidence, given her new status in life.
There is a lot of jewelry from the early days she has not worn in recent years. Her taste in jewelry will evolve/change over time. As it has with me and will continue to do so as time goes on. I personally love her taste in jewelry. It has a sense of "simplistic sophistication" to it. It never over powers her outfits, but compliments. Something to be envious about. I am still of the opinion that this new necklace is a remake or redesign of the 2004 one; but with added stones or pearls.