[HO455]

Just a couple of by the ways. The plastic bows that hold the panels up are fragile and won't take much force when you try to remove them. I have heard that heating them with a hair dryer can help but the fiber board will singe easily.
The sagging of the fiber board is caused by moisture getting in the board and gravity pulling it down. I have never heard of anyone successfully reversing this process. But there may be a interior restoration shop out there that deals in high end cars like Packards or Rolls Royce from the 30's and 40's that could do it. One of the side effects of the sagging is the area of the headliner is increased as the thickness is decreased in areas. So to get back to normal you would somehow have to compensate for that.
I had to source two partial factory headliners to get enough dimensions to build a replacement for mine. The thing I noticed is there was a difference in the dimensions of the pieces between the two sets. Enough to say I don't believe they were die cut but hand cut for each truck.
And finally the front panel was the first panel installed then they worked their way to the last panel in the back. I was ignorant of this fact and started at the back so the front panel was quite difficult to install. It also has the compound bends in the corners which I found difficult to keep from breaking when I removed the original.
All in all it is not a difficult job but the factory materials are quite fragile and replacements are not available through the aftermarket.
I hope this helps and good luck!