As no one has given an answer to this question - i thought i would give my opinion of what i think (please correct me if what i have said is incorrect)

the pillar is a representation of where the devil was standing and therefore is just a point where the pebbles should strike to complete the sunnah that our beloved Prophet completed as part of the rites of the Hajj.

The reason that i think the pillar has been changed to a wall is because the number of Muslims now performing the Hajj has increased greatly and therefore the number of people completing this sunnah at any one time is so great that targeting a "thin" pillar must be more difficult then aiming for a wider wall (the pebbles have to strike the pillar/wall) and therefore having it changed to a wall must have been a logical decision by the Saudi authorities.

Hope this answers your question

But hope one of the Brothers or sisters on the forum will correct me if this information is incorrect.

Peace