I would say that if you're going to be replacing the bathroom fan, you're going to be going to and from that location a good few times before you're done. If it wuz me, I'd use a short piece of 2X4 to clear a path to the fan, and then put 2 foot wide by 4 foot long plywood handipanels down (if your attic access allows it) or some 8 foot long spruce 1X6's over the ceiling joists to provide a "walk way" to the fan. You'll have to overlap the boards or handipanels so that both ends are always on a joist.
When you're finished, just remove the "walkway" as you make your way out, using a leaf rake or something to put the insulation back in place as you make your way out.
If you're starting to do this kind of work on your house, you may as well "invest" in a good system as you may be wanting to install pot lights or check out possible roof leaks in future. So, it makes some sense to have some plywood and boards up in your attic to allow you to move about up there without putting a foot through a ceiling.
I own a small apartment block where there simply isn't access to the bathroom fans from above. You have to do the entire job from below. It's do-able, but it's SOOO much easier to do this job when you have access from above.
Also, one of the problems you're likely to run into is removing old duct tape. I use a heat gun to warm up the duct tape to soften the glue holding it to the galvanized duct. When it's hot it comes off easily. You could try a hair dryer, but the installers that work out of any retail flooring store will all have heat guns, and if you ask to borrow one for a day or two, and provide a $50 damage deposit or something, the Installation Manager at that place could probably provide one. Typically inexpensive heat guns come with a Hi-Lo-Off switch, whereas the expensive ones have a variable electronic temperature control. There's bound to be a cheap Hi-Lo-Off type heat gun kicking around the place that they don't use much, and that's all you need.