many thanks for your question. Schema Compare for Oracle is designed to make schema changes without any loss of data. Where possible we use ALTER statements to change table definitions. However, there are certain changes that are not possible with an alter statement. In that case we copy the table data into a temporary structure, before the table is dropped and re-created. The original data is then copied back into place.
There are circumstance where data can be truncated e.g. the 'width' of a column is reduced. In this case we provide explicit warnings in the tool that this happening.
My advice would be to try out Schema Compare for Oracle for yourself, but always make sure you've taken a backup of the target database before you make any changes
Tom Harris, Red Gate Software