my 69 Cutlass is pretty much stock and original and I would like to keep it that way. One nagging thing that bothers me is the drivers seat is "saggy" I feel like I could use a phone book to sit on while driving..

The seat covers are in perfect shape, little wear and no rips pulls or tears. I do not want to replace them My concern is if I pull off the bottom cover to inspect the seat frame and possibly have to fix a few hog rings and/or replace the foam seat bottom will I be able to get the seat cover back on without tearing or ripping 45 year old vinyl?

 

It sounds like the foam and possibly the springs are getting weak,you can install new foam and use the original covers but will need steam to soften up the vinyl. Years ago I had to install new foam under the original covers for a corvette and it was hard to do but got it done.

 

My seat where like that in my 65 I ordered new foam and replaced it. I reused the original covers and it worked out fine. I won't say it was easy but it also wasn't terribly hard. I did find 1 broken spring in the seat side but I repaired it at the same time.
The best thing to do is wait till spring to do it and lay the old seat cover out in the sun and let it get good and hot before installing it, I also absolutely soaked my in vinyl dressing and let it sit in the hot sun to absorb it and soften up the vinyl as much as possible before starting.

 

You can also use spray Silicone on the foam rubber and the vinyl will slide over it much easier.

 

I placed a plastic bag over the seat, slipped the cover on, and then pulled the bag out (you'll probably shred the bag but that's the plan). You'll also have better luck if you do it out in the sun in the summer.