To remove the old ones heat them just enough to soften them with a Butane Ligther or Candle a d yank them off. If you over heat them the Plastic melts and glues itself to the Barb.
I cannot help with the rest as I replaced mine with 1/4" Rubber Fuel Hose and clamps.

There is one thread where someone replace the Plastic Tubing with the American version of the tubing which is yellowish in color. The issue was that tubing had to be heated and shoved onto the barbs.
For myself I was not sure how you would tell how much to heat it. If over heated it is not going to seal.

On another forum a member just went to the Hardware store and picked some clear plastic tubing and shoved it on. He did no research to see if it was something correct for the application he was using it for.

__________________
84 300D, 82 Volvo 244Gl Diesel