Any reaon why you want to replace them? If you buy replacement you still have to weld them into the downpipes, in which case you should check the replacements have non stainless end caps otherwise you will struiggle to weld onto a mild steel exhaust. Is it worth the hassle, I think I have seen new down pipes with flexis for £60 something each.
Other thing is if they are not leaking I wouldn't bother and save your pennies and buy down pipes when they leak.
I used to design Flexis for a OE exhaust company, the outer braid which is probably hanging off by now is mainly for stone protection as the bellows are 2-ply construction of 0.3mm stainless layers. They will still take a fair old knock before they hole. the braid also acts as a movement limiter so the flexi isn't over loaded, but so long as your hangers are still doing their job again you will struggle to note the difference.
I spent 6 years tuning outdecoupling NVH exhaust problems with flexis. With hindsight its funny, We took months of test track driving and design to get the correct vibration results and aftermarket exhaust just use a generic flexi with the right legnth and bore. To be fair though the average driver would be hard pressed to notice the differences between exhausts. Note is generally more noticable than a small vibration hear and there.
Was under my car last night the flexis had seen better days but the holes around the lambda sensors decided the downpipes fate. I am going for 2 new downpipes and a cybox or similar cat back. all of the hanger rubbers and an odd exhuast hanger bush just after the rear downpipe had all seen better days so it was no wonder it was rattling so much.