how can I change the contact type for one contact (to/from individual/household/organization)?
Say you want to convert from an individual to a household. One very hack-ish way is as follows: 


Create a new household with the desired household name.
Note the "contact id" values for the individual (INDIVIDUALID) and for the newly created household (HOUSEHOLDID). (You can find this "contact id" value at the bottom of the screen when viewing the contact where it says something like "CiviCRM ID: 2345". Also this id value is in the URL of the page where you view the contact.)
Manually navigate to the following URL where you will merge the two contacts. In this URL you will have to replace "HOUSEHOLDID" with the id value for your newly created household and replace "INDIVIDUALID" with the id for the individual you want to "convert" to a household.

http://example.com/civicrm/contact/merge?reset=1&amp;cid=HOUSEHOLDID&amp;oid=INDIVIDUALID&amp;action=update
Check all boxes, except make sure "contact type" and any "name" fields are un-checked
Merge the individual into the household.