I think I would knock off the old pads but leave the footers, as long as they are stable and don't tip. Then I'd drill some 1/2" to 5/8" holes in the tops down about a foot using a masonry bit and a hammer drill, good excuse to buy tools eh! At least 4 holes. I'd insert rebar into these holes with some hydraulic cement to hold them.

Then carefully measure the height from the bottom of the rim joist (planned bottom, be sure to allow a little slope on the porch deck to drain water) to allow for the concrete block. I suspect that you will use one regular block and one cap block for a 12" height plus about a 1/2" seam for mortar at the bottom and between, 13" total. Frame up a pad 13" below the planned bottom of the rim joists.

If 13" will put the pad below grade, then plan on just one block with a 1/2" mortar seam so that will be 8.5" below. Bent the tops of the rebar as needed (outward along the long dimension of the pad, and pour the concrete. The pad should be proud of the block by about 2" so it will need to be 10 x 20.

Here is an alternative to consider, there is a precast concrete pier, but it is only about 8" tall. You could follow the above for the poured pad, but 12 x 12" and use it instead. You would not need to mortar it down but you could. The top has groves for 2x and a 4x4 in the center. You can set the block and then either run a short 4x4 PT up to the level of the rim joists and bolt them together (do not nail, use good bolts) or If they are at the right height, you can just lay the 2x in the grove. Using the 4x4 PT gives you some wiggle room to make sure the deck is perfectly level and this could be the easiest way overall.