Starting in Tiki18, it's possible to change field types for those without a risk of data loss or corruption. For the risky ones, you still need to activate the "Change Field Types" preference.
Starting in Tiki12 a "Change Field Types" preference (default status: not activated) has been available on the Admin Trackers page (tiki-admin.php?page=trackers). When this option is activated, a selector is displayed in the "Advanced" section when editing a tracker field. Needless to say, if the tracker field already contains data, there is a chance the data will be lost or corrupted if the field type is changed to one not compatible with the data. As the warning in the field edit dialog states:
Changing the field type may cause irretrievable data loss - use with caution!