El Capitan clients / Mavericks server

Hi -

The server environment at my company is Mavericks/10.9.5. We've been holding at this OS version to retain some web facing reports for our customers that aren't supported in later versions (relates to Filemaker's Instant Web Publishing).

We recently purchased a couple of newer laptops that shipped with El Capitan/10.11. All has gone OK except for our OS X Server Calendar service that's hosted on the Mavericks server. Basic problem is that users can't add invitees (fellow employees and server account holders) to events. I imagine this is just the tip of the iceberg and that there will be numerous other issues related to Users/Groups/Open Directory.

Is there any way to remedy this? I had hoped that the newer OS would be "backward compatible" and talk to our older server fine...

Thanks...