You may try this instead:
# Advanced Rule Set
RewriteCond %{QUERY_STRING} var=foo [NC]
RewriteRule ^path/index.html/? http://newhost.com/Page?pagename=form1000?id=hello [R=301,L,NC]
RewriteCond %{QUERY_STRING} var=bar [NC]
RewriteRule ^path/index.html/? http://newhost.com/Page?pagename=form1000?id=byebye [R=301,L,NC]
RewriteCond %{QUERY_STRING} var=xyz [NC]
RewriteRule ^path/index.html/? http://newhost.com/Page?pagename=form1000?id=world [R=301,L,NC]