I'd say 50% chance that the problem with the 6100 is the battery.
What happens when you try to start it up?
You can get a new battery at RadioShack for $8 or so.
The Centris uses the same battery I bet. Try swapping batteries before you go through the hassle of swapping HDs.
Even if it doesn't use the same battery you may find that leaving the 6100 plugged in overnight will charge up the battery enough to let it start up (if that is, your 6100 has been left unplugged for a long time).
Removing the HD is rather easy from a 6100.
What part are you having trouble with?
I haven't done this for a couple of years and this is from memory:
Open the case by lifting up the the snap tabs in the back (6100 is great this way -- every thing is right there when you open the case, in contrast to 7100 or 8500 etc Apple didn't get it right again until the Blue and White G3).
Blow the dust away
Prefereably with a can of compressed air.
Disconnect the SCSI cable from the HD.
Disconnect the power cable from the HD.
Push in the tab so that the sled slides out.
You did it.
Now if the sleds match you can just swap the sleds and the drives together. If the sleds are different you'll need a Philips head screwdriver for the four screws...
Hope this helps.