Once a field is created, the properties that allow to be changed are limited. Changing the data type is not allowed. However you can always change the field name and a few more properties depending on the data type. The properties that do not allow further modifications are displayed as dimmed.
You are allowed to rename/add or insert options of a field of data type option list.
You are allowed to change the lookup fields of a field of data type relationship.
Tip: If you decide to change the data type of a field at a later time, export data on that field for all records to a file,
create a new field with the new data type, import the file back to the new field. Beware that this will work only if the
exported data and the new data type are compatible. e.g. you may convert most of other data types to "small text"
but you may not convert any data type to "option list" or a numeric field to "date".