This kind of backwards compatibility should certainly be tested.
Yes, that sounds right.
More importantly, it would be very useful if when we remove the code
from devel/sage/sage/server that is needed to migrate, we include an
error message that says: "Please migrate this Sage notebook server
install by installing a version of Sage between 4.3 and 4.8,
Incidentally, I also plan to support the format of the sage notebook
server files from 4.3 indefinitely. It doesn't involve any version
specific pickles, etc.