Online conversion procedures avoid the need to generate a copy of an entire table in order to convert the data type of a column in the table. To convert a table's column to a new data type, a replacement column is created in the table, the replacement column having the new data type. A background process dedicated to converting the column translates values in the column of the old data type to the new data type and stores the converted values in the replacement column. In addition, as transactions of users are applied to rows in the table, the value in the column of the old data type in the rows is converted and stored in the replacement column. Once the background process completes, the replacement column is used in lieu of the converted column.