Nope - not happily
You can use older versions of agents (usually one or two revisions) but not newer
If you want to keep using 11d but can olny buy a 12 license then you'll need to contact Symantec Customer Care (who are surprisingly efficient) to get them to give you a previous version code (if the one you get with purchse doesnt work on 11d anyway - cant remember if it will and dont have one handy to check)
If you had up to date maintenance on 11d you could update to 12 for free and save yourself the hassle as well as getting more device support and features such as AFO and IDR bundled in
