It’s not just a case of “roleplaying wise”, Yeol, but usually a case of track speed too – that high speed track is very sensitive to sharp turns, to the point of being no better than the basic track if you turn too sharply…
So, given that you likely built that old track when you had no money to spare and speed wasn’t an issue so much as not bankrupting yourself, there is a strong technical case to be made for relaying HS track rather than a straight replacement.
I’m not saying that I’d not like to see a straight “upgrade this line” option, just as I’d like to see upgrading a loco or rolling stock not zero the passenger/cargo count, but it does make more sense to lay a new line for high speed traffic.
Unless you’re playing on Hard, in which case… how are you making money with trains?? 😛