MY pennyworth is that by 10 years, most octopi are showing their age, and by 15, they are an odds-on bet to fail at the least convenient moment. Must check mine, along with many other things that will go on a 15-year old car.
That said, unless you've had experience of replacing more than one octopus of either variety (large or small) before, it's kind of hard to drum up enthusiasm for replacing one as a preventative measure.
If the engine has to come out, or you're jacking the engine up off the upper mount, or if you're removing r/h driveshafts, then it becomes easier to do. Engine-out octopus replacement must win by a mile, though - access is no longer a problem.
Useful tip: where it's possible, keep the old pipes in place, and borrow the short length of clear plastic tubing from the tank vent. I can't remember the exact procedure now, but this helps when threading new pipes through the protective sheath located next to the inner wing. I seem to remember it's a case of cutting the old pipes below the sheath, joining old pipe ends to new using the press-on ends of the vent pipe, and then pulling the old pipes through with new one attached via the vent pipe.