Mapping of values between two different type of variables is not allowed. Only the same type variables can be used for creating "item variable assignment" - IVA record.
After creating IVA record, it is possible to change the type of source variable. However, it is not recommended to change the variable type after configuring "Item variable assignment" as it may have an impact on the data and data type at the database level.
One example is when "Item variable assignment" is configured between two text type variables and then the type of the source variable is changed to List collector, the target string variable shows the actual value (sys_id) instead of the display value.