We want to replace the boards on our front porch but don't know how to raise the roof to get under the supporting columns. The columns are sitting on little metal supports. The boards are tongue in groove but we don't necessarly want to replace the boards with the same lumber. Can you help?

Don't try to raise the columns. Cut the present floor at the base of the columns, put in the new floor and use quarter-round molding around the base of the columns, of the same wood as the new floor.

Chances are that you have 4x4 post and 1x4 or 1x3 tongue and groove porch boards. The brackets are screwed to the bottom of the post and have little prongs that dig into the floor for stability. Most of the time the post will sit on two boards. Insert a pry bar under the bracket on one side and lift the post, lift the post high enough to remove the prongs from the board, about 1/4". Remove that board and replace. Move to the other side and repeat to remove the other board. Be careful not to break the foot. They are cast metal and will break.

If the wood under the posts is not soft or rotting go with robkelly2242. Will be easier for you. IF the wood under is not in good shape follow the advice of hkstroud. Leaving bad wood under the post would cause problems.
Mike