What kind of information I may lose ?
Can I modify my file on Oracle Report 11g and compile it in Oracle Report 10g ?
Can I modify my file on Oracle Report 11g and compile it in Oracle Report 11g and run it on Oracle Report 10g ?
Question 1 - the file will no longer be useable with Reports 10g - you will loose everything.
Question 2 - No, newer versions of Oracle Forms and Reports are not backward compatible with older versions.
Question 3 - No, see answer to question 2.