If you have selected a range before applying an operand conversion command, IDA will display a dialog box.
You can choose a range of operands to perform an en masse operation:
The operation will be performed on all operands as a toggle. For example, if you ask to convert to a character, then all non-character operands will become characters, and all character operands will become non-chars.
The operation will be performed on the void operands which contain immediate numbers in the specified range.
This selection will convert all operands with the specified type to undefined operands. Example: all characters become non-characters.
This selection allows to convert all operands that do not have the specified type to the specified type. Example: all non-characters to characters.
This selection allows to convert all operands without any type to the specified type. Example: all operands with no type to characters.
IDA will check whether an operand can be represented with the specified type (as a character constant, for example), and perform type conversion only if the check is successful.