I went through all this when I bought the TTX for my 2010. I have copies of the spec sheets here for both shocks (older TTX and newer). There are some differences in length, head, and end eye, as well as internally being an updated design with different valving specs. Let me know if you want more details. I never physically confirmed if they would fit or not. Based on the information below I never bothered looking into it any further.
GG11820 - Gas Gas EC250/300 2002-2011 TTX44
Length: 427mm
Stroke: 143mm (125@10000N)
Head: 03482-04
End eye: 01283-59
GG12820 - Gas Gas EC250/300 2012 TTX44
Lenth: 463mm
Stroke: 145mm (125@10000N)
Head:03482-41
End eye: 01297-64
I was also under the impression that Ohlins Sveeeden is no longer manufacturing the GG11820 shock. I bought mine through Skeeters Motorsports, who supposidly had bought the remaining 5 shocks from Ohlins USA. I can't remember how much I paid for it but pretty certain it wasn't much cheaper than I could get a GG12820.