Just finished my conversion (on an '06 FZ6). It wasn't too bad at all. An accident causing the destruction of my original gauge prompted this.
I purchased a cluster from a 2012 FZ6R (they are the same as the 2nd gen FZ6 clusters). The FZ8 and FZ1 clusters should also work, the FZ8 one just has a white face instead of black.
Since the 1st gen FZ6 cluster has two connectors on the back, and the 2nd gen cluster has 1 connector, the easiest thing to do is to get the front section of the wiring harness from a 2FZ6 or FZ6R. The 2 connectors on the back of the harness that connect to the main harness are the same in the 1FZ6, 2FZ6, FZ6R. Your 1 connector now works with your new cluster. It's plug and play. Everything worked exactly as intended on my new gauge. The only thing is your odometer will be incorrect, naturally.
my bike now:
new gauge, it has one 16-pin connector on the back
updated wiring harness sections you need to get, note the black connector in the middle on the bottom, that's the 16-pin gauge connector
here's the two back connectors that connect to the main wiring harness, they are the same on 1FZ6 and 2FZ6
even the service manual and wiring diagrams show that they are pretty much interchangeable....