is it possible to only change the ball bearings on the

You would just replace the idler pulley completely.

Those pulleys are molded around the bearing. Replacement is the only option. OEM or aftermarket, I suggest the aluminum ones found here in the forum.

Thanks for reply, do you have a link to the alu wheels??

Actually I did replace a bearing only with a equivalent bearing from a local bearing supplier. There is a molded ridge that keeps the bearing in the plastic pulley. I put the pulley in my lathe dressed the lip just a bit, pressed it out and the plastic flexed enough to allow it out. The new bearing pressed in secure. IF too much of the plastic is removed you could still consider locktite to hold it in place.

I also found the OEM part had a "made in china" bearing so I bought a replacement from Auto Zone and checked it. It had the same made in china bearing. It was installed for about 40% the cost of OEM and worked for 40,000 miles until I sold the car.