The change is in how object data is stored. When you write an object to a savefile buffer, it creates a directory for the object, and calls your Write() proc in that directory. When you return, it used to pack up all the data in that temporary directory and write it into the original buffer where you put the object. It now skips that step and simply stores the name of the directory in the original buffer. That makes the writing faster and it also makes the reading faster, because it doesn't need to recreate the temporary directory every time you read in the object. In other words, the Write() directory is not temporary anymore but is persistent. It has a special name (beginning with a ".") to avoid conflicting with any user-created directories and is automatically deleted when the data in the associated buffer is deleted. It should also be hidden from you when you look in the savefile.dir list, so if everything goes well, you can pretend you don't even know about it.
I don't mean to scare anybody with this detailed description. As far as you are concerned, things should work as before except faster (and in some of my tests, about 97% smaller!). I just mention it so you know why things are different and why you should rewrite your savefiles from scratch so they can be converted over to the new format. (You can read in old data and write it to a new file to accomplish this if you don't want to lose anything.)