Re-cabling an S5

Hey everyone,
Fair amount of stuff on internal cables out there but I’m looking input on re-cabling an S5 specifically. The bike already has cables in it and everyone says use the existing cables to thread the new ones through, but the problem I’m foreseeing is this: Those plastic liners that run from the entry point on the TT to the down tube seem like they would interfere with the threading of a cable that is glued, tied or whatever to the existing cable. Anyone have any work arounds? I’d really like to not just thread the wires without a guide as getting them out the hole by the BB when the BB is in place is a PITA.

Related question: those plastic liners, do those require replacement at any point? If so, where do you get it?

Thanks in advance