Comment 4 for bug 561599

Any filesystem type change will cause the partitioner to reformat. If you want to reuse an existing partition, then you must be careful.

Aside from fixing whatever the autodetection problem is (which merely moves the problem around, since you still have to know what filesystem is in use and many people won't know offhand), the only way I can see to improve this is to show the current filesystem type somewhere. However, that code is subtle and quick to anger, and I don't intend to touch it any more for Lucid at this point lest I introduce worse problems.