Extending Travel on a Rockshox Tora Trail 289?

I currently own a silver Rockshox Tora trail 289 (non u-turn) with 100mm of travel.
I have been looking at diagrams of the fork and looking at the parts available. I have found that it would be possible for me to buy a 120mm spring for the fork and a 120mm coil shaft and negative spring for about £25. Im wondering if there would be any problems if i was to replace these 2 parts during a service to increase the travel.
Any help would be greatly appreciated on advice of more parts needed etc.
Links:
the following link is the technical manual and the parts list and diagrams can be found on pages 12 and 13
https://www.sram.com/sites/default/files/techdocs/2011%20RockShox%20SPC_Rev%20B.pdf
I have been looking at diagrams of the fork and looking at the parts available. I have found that it would be possible for me to buy a 120mm spring for the fork and a 120mm coil shaft and negative spring for about £25. Im wondering if there would be any problems if i was to replace these 2 parts during a service to increase the travel.
Any help would be greatly appreciated on advice of more parts needed etc.
Links:
the following link is the technical manual and the parts list and diagrams can be found on pages 12 and 13
https://www.sram.com/sites/default/files/techdocs/2011%20RockShox%20SPC_Rev%20B.pdf
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