I can understand why you might need new tracks for bullet trains (old ones not perfectly level and aligned, quality of materials, friction properties, whatever).
But why would you have to build new _stations_ for them?
You need new tracks as in completely different right of way. Bullet trains need very straight tracks, existing tracks are too curved and at high speeds you'd fly off in the corners (tilting trains exist but have limitations). And if you're building new track, there are no existing stations to reuse.
Generally, when you go the high-speed route, you have lines that are a lot longer than slower, more local lines. You do not want stations along the way to be terminus-style stations, where you'd
1. have to switch out the driver position from one end of the train to another end to just leave the station again and
2. have a series of track switches which may malfunction (which ironically is a challenge even for highly industrialised countries like Germany), and it
3. means more and slower track to be maintained.
If you set up a bullet train program, you want speed, and these problems kill the speed advantage. The solution is to build a new station in which the train can just pass through.