I started fly fishing in 2001 . . I have since collected about 13 reels with a few additional spools . . . The oldest lines are finally being replaced. I have dacron on the lighter weight line/reels and gelspun on the larger stuff (6 wt and above) to get greater capacity in case I ever need it.
I have never changed mine and some of my reels go back to 1995. I have thought of doing it, but haven’t worried about it because I don’t leave my reels in the heat of a car or exposed to sunlight unless fishing…
Just my opinion, your mileage may (and probably will) vary.
When I got back into fly fishing, I purchased new rods and reels at that time. I seldom go far down into the backing even with LMB, stripers and hybrids. Periodically, I peel it all line and backing off the reel, check knots and give the backing a couple of heavy test pulls and stretches. So far, it seems fine. Now if I were going for bonefish…
BTW, I restarted serious fly fishing in 2001 so our stuff is right about the same age. Mine’s fine and yours should be too. 8T