Some of these updates are clearly hacks, in the sense that if you did not have to worry about backward compatibility, they would certainly not be implemented in that way.
They're not like upgrades from ext2 to ext3, it's like ext2 was modified in a weird way to support new features that older ext2 implementations do not know about.
Hard Links for example are implemented by adding files which look a lot like symblinks. As soon as you create hard links to a file, the file is moved into a invisible folder named "Private Data" and becomes a "node file". The original file is replaced by a stub containing the CNID (special type, special content). That's not as efficient as having iNodes and hard links from day 1. If you mount such a volume on Mac OS Classic, the "Private Data" folder is totally accessible and modifiable by the user, and of course hard links do not work.
Hot file clustering and journaling are implemented in similar ways to hard links (invisible folders).
The opposite example are extended attributes (xattr) which were added in 10.4. A special filesystem structure named the "Attributes File" was reserved in HFS+ from day 1 but never used. De-defragmentation does not rely on any changes to the volume, it's purely done in the Filesystem driver stack.
Things like Copy-on-Write or Snapshotting, are probably impossible to implement in a backward compatible manner though...
