You can convert from Text Area (Long) to Email, Phone, Text, Text Area(Rich), or URL without loosing the existing data. But linebreaks in Text Area (Long) will not show in the Rich text field.
Solution: Update the Text with the apex code Text.replaceAll('<','(').replaceAll('>',')').replaceAll('\n','<br/>');
Note: You can only convert rich text area fields into long text area fields.
If you are using this Long text field in an Apex class or trigger, or in Visualforce page saved using Salesforce.com API version 17.0 or earlier, you cannot convert to a rich text area field.
no data will be lost! but vice versa will do the damage! let me know if this helps you!
Solution: Update the Text with the apex code Text.replaceAll('<','(').replaceAll('>',')').replaceAll('\n','<br/>');
Note: You can only convert rich text area fields into long text area fields.