change value field type value

change value field type value

Can I change the field type value of a custom created field?  For example, I created a picklist and now i realize that I should have done it as a multi-picklist.  Is there a way to simply change the type or do I have to create a new custom field?  If I have to create a new field can I (1) copy the values I have already added (do i have to retype each one) and (2) does the deleted custom field count against my 150 ?