I have just replaced the standard shock on my B1H with a virtually new one from a 2012 bike and thought I'd share some useful information.
Beforehand, I did a lot of searching to see exactly what additional parts were needed and what difference it would make to the geometry of the bike, but couldn't seem to find any definitive answers.
I bought a shock, plus entire linkage and measured everything up. Both shocks are 338mm long between the centre of the top and bottom mounting holes. The linkages are virtually identical within a mm or so, therefore, there should be virtually no change in the geometry of the bike. The main difference is that the mounting gap at the bottom of the 03/04 shock is 32mm and only 30mm on the 09/12 one.
After removing the old shock, I went to remove the linkage arm, but the bolt holding the linkage to the frame wouldn't come out without removing the exhaust downpipes. So after looking at both linkages and measuring them, I discovered that I could leave the existing linkage in place and just swap the sleeve and 2 oil seals (circled below).
The main uni-trak arm part of the linkage is exactly the same width where the shock bolts on, it's only the sleeve and oil seals which makes it 2mm wider on the B1H. These only cost about £15 to get the 09/12 parts. The added benefit of keeping the B1H linkage arm is that it has grease nipples, so you don't have to take it apart to grease it up.
I'm guessing that there is similarly very little difference with the 05/06 and 07/08 shocks/linkages as well as the above.