I have a date field which I set to Extended but decided after testing a couple of records to change it to Standard, however, when I click the Standard radio button all that happens is the definition window goes blank, if I then click the Type radio button it brings the definition back up but it still says Extended, there is no way of changing it.  Is this intentional to prevent errors, its not clear? 

There is only one word for this....

BUG!

The field definition dialogue is unnecessarily over-engineered and as  result there is some peculiar behaviour (try to switch to and from Lookup field etc).

This problem has been traced back to some changes in Summary Fields in 8.1....