Lots of Python systems have tried hot-fix and resume. They don't work very well, though some may get close. Some of the problems are actually quite challenging regardless of environment -- you have to take each object and figure out a way to update it according to new specifications. So you either have to specifically specify how to upgrade each object (which is burdensome and is more code that can contain bugs, especially burdensome when you are just doing active development and have no valuable objects).
In Smalltalk, sure, you can add another instance variable -- but they all end up as nil. That very well may be an impossible situation except when updating an old version of the instance variable. So how do you deal with that? Allow for the possibility of nil, even though that's a transient possibility? I'm sure there's tools or pattersn to deal with this, but it can't just be automatic in the general sense.