||Sometimes people do in fact want to change the partition type. It should be made clearer that the effect of this is to format the partition, which is bug 262504. One reason I am reluctant to make the UI flow the other way round is because that would be at odds with how the alternate installer's partitioner does it, although that does not necessarily mean it's impossible or even a bad idea. However, I don't want to get into that in this bug. I think it's quite sufficient for this bug to be about the poor handling of setting a mount point on an existing partition, and that the matter of how "Use as:" interacts with the format checkbox can be handled in bug 262504. Please respect that so that it's possible for us to deal with one bug at a time in a reasonable way.
I suspect that what we need to do to fix *this* bug is to rearrange the UI to split up the notion of the intended use of the partition ("method") from the intended contents of the partition. It would then be possible to display "existing ext3 file system, not used in this installation" from "existing ext3 file system, mounted somewhere". For file systems you can sort of infer this from whether a mount point is set, but it's a lot less clear for things like swap areas so I think it would be better to explicitly separate them.