What I mean is transforming I-frames into equivalent P-frames (or even B-frames) that decode to the exact same pixels. The transformation might result in something that is bigger than a directly encoded P-frame (which can tolerate some small errors) but I suspect the new P-frame will be smaller than the original I-frame.

There is no restriction that P/B-frames only reference I-frames, so you don't even need to touch those frames.

For conversion back to MPEG2 it would be ideal to detect or mark the original I-Frames, so you can convert them back (a simpler transformation). But you could also pick any random P/B-frame and convert it to an I-frame with little issue.