I have googled the crap out of this and can’t find an answer.

I have aquired a set of Rockshox Recon Silver TK forks. I read everywhere that you can adjust the travel by changing the spring assembly. So I have found where to purchase the sring assembly from and i now know how to change the spring but is it just as easy as taking the old 100mm spring out and installing a new 120mm spring?

Any help here is most appreciated

What year, air or coil? Coil, im pretty sure (but could be wrong) no but with air you should be able to remove a spacer to increase travel.

Coil. 2011.

Had a quick look but can’t see anything positive sorry. Coil are usually fixed travel unless u’turn obviously. You could double check with tftuned.

Yes, I changed mine to 120 with a firmer rate also. Hardest bit was removing the circlip on top leg for the lockout.
Can’t remember the specifics now but once lowers are off it was straightforward to replace the assembly.

http://www.sram.com/sites/default/files/techdocs/2011-recon-gold-technical-manual.pdf
http://cdn.sram.com/cdn/farfuture/BtRI-fZQaQfTJijnIsbNEA04QJAQL0ilYpvL2wZnIgQ/mtime:1372788151/sites/default/files/techdocs/2011-recon-silver-technical-manual.pdf
http://www.sram.com/service/rockshox/7,104
These normally help