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).
Also, those pipes are pitched specifically at the angle that you see them as part the the overall design of the system. You could really mess things up by rearranging those pipes. I would look for a new place for the treadmill.