I think that it's so usual to lost values  when you submit your form and change request .
Maybe it's better , use of Ajax submit, instead of submitting form when you change the value of dropdown  . it cause to send a request to server and change a part of page  which you want but not your whole page . for doing this that's enough to use form tag of Ajax for JSF lib(a4j) and add
" ajaxSubmit="true" "
to your form tag in the page.
for more information refer to A4J lib refrence.