ok, i did the battery replacement/upgrade today on my iriver and on a friend's. instead of typing this here and in a PM to whataboutbob, i thought i'd just post it here.
whataboutbob wisely suggested shaving off a tiny bit of the molex connector bottom "lip." instead of doing that, i actually just used the molex connector from the original iriver battery... the molex connector from the iriver battery does not have as big of a bottom "lip" so no shaving was necessary.
also, i used a very tiny flat-bladed screw driver to gently unplug the original battery and to plug in the new battery as well.
don't forget to reverse the wires. something i noted was this: while looking at the top of the molex connector (where the little "flaps" are on top), the red wire goes on the left side and the black wire goes to the right side of the connector.
the operation took longer than i anticipated, but basically it took me a long time to take off all the molex connectors and put them on the new batteries. (i was extra careful, better to take a few minutes longer and not damage them.)
hope anyone who replaces/upgrades the iriver battery finds this info useful.