Not that common in my experience replacing all hard brake lines with flexible, most tends to go like for like.
I personally don't see any benefits changing the hard line to flex. It's easy enough to fit hard and the route / mountings are in the correct place to support the hard line, something you may need to change to support the flex, as its flexible !
You do have an investment in tools (flaring tool) and the cheap ones don't work well, so expect £35+ for this tool. Automec sell ready made kits for vehicles that replace all hard pipes, ready to fit, but no direct experience of these also.
But can't see a reason why not if you want to go flexible, it may even be easier to install. I always DIY my own hard lines and it's not difficult with the correct tools.
Yokohama Geolanders, Sony head unit, NAUTILUS Air Horn, DRL conversion, Rear cargo space, Elvis Bobblehead, transfer Guard, Indian hanging Elephant, Koni Heavy track dampers, Custom SS exhaust, Voodoo Doll, Adventure Rack with LED ight bar, vintage ERIBA caravan usually attached (yes it's slow)
