I would use a rubber or pvc expansion joint, but would definately dowel them together for vertical movement.. What is your area of the country? These type of connections are notorious for moisture problems in areas with high water tables. Is there going to be a wall framed at this joint? If so, be sure to get professional insect treatment; these hidden joints are like sounding the dinner bell for termites.Seems there are differing opinions?
A new slab will be formed up against an old stem wall. Should I dowell it in (maybe #4 rebar?) or use some sort of expansion joint?
What's the best practice?
Correction: remove red and add blueDisimilar methods of construction never get doweled together. (Frost protected don't get doweled to non-frost protected, or slabs on grade)