Values must not exceed (or fall below) the size limit set for the field.
For example, you can convert a Currency field to an Integer field only when those values are greater than 255 and do not exceed 32,767. Access tries to convert other values, meaning that you see underlined text, and the cursor changes when you point at the link, but the links don't work.
In some cases, the conversion process might delete some data.
For more information about restrictions when you convert data types, see the section Restrictions on data type conversions.
You cannot undo the results of an update query, and making a backup helps make sure that you can reverse your changes if you change your mind.
When you need to update data from one table to another, consider the following rule: the data types for the source and destination fields must either match or be compatible.
the UPDATE statement is especially useful when you want to change many records or when the records that you want to change are in multiple tables.
Otherwise, Access appends to the beginning of each value, and the resulting links are not valid.
The dates that you can convert depend on the size of the number field.