How will changing the mysql field type from INT to VARCHAR affect data previously stored as an INT

Get your MySQL server into strict mode before you change the column type and make sure that your varchar(n) column has a large enough n to hold all of the integers when they’re converted to strings. If you’re not in strict mode then MySQL will silently truncate your data to fit your string size:

If strict SQL mode is not enabled and you assign a value to a CHAR or VARCHAR column that exceeds the column’s maximum length, the value is truncated to fit and a warning is generated. For truncation of nonspace characters, you can cause an error to occur (rather than a warning) and suppress insertion of the value by using strict SQL mode.

But if you get into strict mode first:

mysql> set sql_mode="STRICT_ALL_TABLES";
mysql> alter table table_sample change col_sample col_sample varchar(6);

You’ll get a nice error message like this:

ERROR 1406 (22001): Data too long for column 'col_sample' at row ...

if your integers don’t all fit in your varchar.

And, of course, you will have a fresh verified backup of your database before you try to change the table. And by verified I mean that you have successfully restored your backup into a test database.