Range of possibilities. Most likely would be a packaging change. Next most likely would be a change in cache sizes. Next most likely would be process variations - using a newer process with an older design, for example. Less likely would be changes in SoC blocks - thinks like thunderbolt on board. Conceivably additional or changed instructions, but only if they are microcoded variations.
I'm guessing that we're talking special packaging, if anything. Intel isn't desperate like my former employer was back in the day when we were offering to add major changes to the instruction set for big customers.