I have a pair of 06 Reba SLs which I have just, today extended to 100mm.They were OEM from a Giant XTC and originally set at 85mm.
I too was told that OEM forks were often,fixed travel,but on disassembly I was glad to see,2 x 15mm spacers in there.
I have reassembled to air spring,and am now just waiting now for a new shaft bolt so I can reassemble the lowers.
This was the first time I had disassembled the forks,but a few good reads of the RS Reba service manual and watching the tech videos,have made it fairly easy to follow so far.
Was a bit of a fiddle to remove the snap ring on the air spring but managed OK.
:D