I recently put a fantastic fan on our Hi-Lo. We LOVE it and I highly recommend it, or one like it, to anyone.
Getting the old caulk off was the hardest part of the job. I started with a regular putty knife but it was too flimsy, so I had to go buy a strudy scraper blade. I don't know any tricks for getting the old caulk off other then scraping and tugging on it.
*Be careful with the scraper that you don't gouge a hole in the roof.
*Take your time so that you get all the old caulk off, else you could end up with a leak.
*Follow the instructions with your fan to properly seal it.
It's really not a complex job, it's just getting the old caulk off. I posted my experience installing mine, along with some pictures. Hopefully it will help. Have a look at this:
Installing a fantastic fan
It's worth it, don't give up!