Yes, in fact, we used to convert ext2 to ext3 with one single command:
tune2fs -j /dev/<device>
This adds a journal to the filesystem, making it ext3. In fact, you could mount ext3 as ext2 (you probably still can). It would just not replay the journal.
The same is pretty much true for ext4, it primarily consists of extension for ext3, but forked as a separate filesystem in order not to destabilise ext3. You can mount an ext2 or ext3 filesystem as ext4. You can mount an ext4 filesystem without extents as ext3.
It should also be noted that ext2 is strongly influenced by UFS1 (FFS), which has been around forever.
ext[234] is not a modern filesystem. However, it is very well understood and stable.
