I think older cars without HW3 are stuck with EAP pretty much as it exists now, plus the new Summon. No new computer. Minor improvements in the future.
HW3 cars could be an exception. Basic AP could run on HW3 as a crippled FSD. That would give Tesla a more unified software build, with AP/FSD enabled as a simple software flag. That would provide better safety as well, and a smarter AP, with better object recognition and driving behavior. That makes sense to me, and was my case for waiting for HW3 or upgrading to FSD even if you didn't care about FSD. But maybe Tesla will maintain completely separate software builds for AP and EAP versus FSD, maintaining a wider gap between AP and FSD, and no difference between AP/EAP behavior on old hardware or HW3.
With FSD on HW3 I fully expect basic AP (when you are driving) to use FSD software. That should handle (eventually) all the cases we'd like AP to handle now, and provide better emergency braking, but allow you to control the car as much as you want. Tesla could use the old limited AP software, but why bother?