I am presuming that at this late stage in development, the default file system of 11.10 will not change from now until final release. Will it be ext4, and if/when it changes, how will systems on the old format upgrade, in their current state, or changing the entire parition?
Oneiric is past feature-freeze - there is no chance that the one of the fundamentals such as the file-system will change at this stage.
Given past experience of LTS's - these tend to be very conservative - i.e. incremental change - thus very unlikely to again change file-system - again many people are speculating about 12.10 which could involve major changes such as Wayland.
There was at one time much talk of btrfs being the replacement for ext4. Btrfs is maturing with time - with support gradually being added - for example - update-manager in oneiric now has btrfs support.
As to how we would migrate from ext4 to btrfs - there is already support to convert from ext4 to btrfs by installing the package
Assuming Canonical makes the decision in (for example) 12.10 - expect to see a blue-print which will outline using these packages to provide a smooth migration path.
When I first installed Ubuntu the default file system was ext3. I am now using 11.04 on the same / partition and /home partition and the file system for both partitions is still ext3. An upgrade will not change the file system type.
A new install will change the file system type if you allow it to use the default settings as is proved by a new 11.04 and 11.10 install on two other partitions. The file system on those two partitions is ext4.
It is called being backward compatible.