I've got the 2019 version of the game - which has the newer Portsmouth. So i'm considering grabbing the UK classics pack off the store ($24), mainly for the Liv-Manch route ($20 in itself) and also carries the WCML (which I'll eventually need for free/paywares) and older Portsmouth Dir. Will installing this older version alter the files in anyway of the newer Portsmouth route?
No, the old Portsmouth Direct is completely separate from the new one. The old version installs into RSC<GuildfordDistrict, while the newer one goes into DTG<PortsmouthDirect.