Oracle SQL to change column type from number to varchar2 while it contains data

create table temp_uda1 (test1 integer);
insert into temp_uda1 values (1);

alter table temp_uda1 add (test1_new varchar2(3));

update temp_uda1 
   set test1_new = to_char(test1);

alter table temp_uda1 drop column test1 cascade constraints;
alter table temp_uda1 rename column test1_new to test1;

If there was an index on the column you need to re-create it.

Note that the update will fail if you have numbers in the old column that are greater than 999. If you do, you need to adjust the maximum value for the varchar column

Add new column as varchar2, copy data to this column, delete old column, rename new column as actual column name:

ALTER TABLE UDA1
ADD (TEST1_temp  VARCHAR2(16));

update UDA1 set TEST1_temp = TEST1;

ALTER TABLE UDA1 DROP COLUMN TEST1;

ALTER TABLE UDA1 
RENAME COLUMN TEST1_temp TO TEST1;

Look at Oracle's package DBMS_REDEFINE. With some luck you can do it online without downtime - if needed. Otherwise you can:

• Add new VARCHAR2 column
• Use update to copy NUMBER into VARCHAR2
• Drop NUMBER column
• Rename VARCHAR2 column