Get a 4x4 longer than the height of the archway. With the old or column to be replaced still in place, position next to that column the 4x4 and with it at an angle drive it straight to take all the stress and weight of the old column. this should be high enough to allow the old column to come out easily.
Remove the old column and have the new one ready. Make sure the base of the new column has been diped in purble wood preservative.
Now position the new column in the old space, it should go in rather easily and when the 4x4 is removed, the pressure should be enough to make the new column nice and tight.