I am looking to purchase a treadmill to use in my basement hut the location I want to use it has radiator pipes in the way. I am wondering if it would be possible to replace the pictures section of pipes with ones that are more flexible / smaller that would keep them out of the way.
You don't want to decrease the size of those pipes. The correct solution is to re-route the pipes to be closer to the ceiling or inside the joist bay. You probably don't have the tools to do this yourself, and it's not really a DIY project anyway, unless you happen to be a pipe fitter (in which case you wouldn't be asking this question).