Upgrade my alienware 15 r3 60hz gsync to 120hz gsync ?

Why not ? The only difference is the refresh rate, both are gsync enabled

No its not possible.

Hi,

It cannot be done because the motherboard on the 60Hz system cannot support the 120Hz display. Even though they both have g-sync, the motherboard will also need to be replaced in order to do this upgrade. 

Hi may i know why you said its not possible, i contacted the official alienware facebook page about this and they asked me for my service tag number, they told me that it is possible and that i would need to purchase the parts. I am confused now. 

We apologize for the confusion, I consulted this with our engineering department and they confirmed the motherboard does not support this upgrade, therefore, you cannot just replace the LCD because the system is not going to properly recognize it. 

how much would it cost to have the board and the screen replaced ?

Unfortunately, we do not sell these parts alone from our end, but you can contact the Dell Sales department so they can confirm this for you. 

A short research of the intarwebz reveals the short answer; yes.

notebook review forums threads/alienware-17-r4-60hz-1080p-display-upgrade-question.804532/#post-10522010

since you're 1080 (FHD), you'll need to replace the video cable to the 40pin to upgrade to either UHD or QHD. 

The long answer is if you don't already have GSync, you *will* need to other motherboard revision to add it.  In any event, if you have GSync and get a panel without it looks like it'll just be whatever standard refresh rates the panel supports.  Likewise if you don't have gsync on your current system and install a gsync compatible panel, it'll just run whatever standard refresh rates the panel supports with gsync off. 

You do not need to replace the motherboard to change the display panel and it looks like any of the three panels can be installed into any revision of the R4 with the only caveat being the 40pin cable.

4k panels look to be readily available for under $100, but the UHD's are pretty hard to find.  Likewise the 40pin cable runs about $12 or less although some vendors are charging upwards of $100 for it.

40pin cable part number WTNR3

no idea about the 15" panel part numbers - I've got a 17