56d is compatible with old scripts, of course to jump from an 8 year ago 56b to the latest 56d you can't just update your SphereSvr.exe, there's some root scripts that must be updated too (sphere.ini, sphere_defs.scp, sphere_msgs.scp, spheretables.scp, ...) but in general all triggers/functions/etc still the same
depending how deep is your server customization, this can be an simple copy/paste overwriting all scripts or even an manual work of many weeks, but this is only valid to root scripts. You doesn't have to rewrite all your custom systems, custom items, etc. Unfortunately custom changes made by users on root scripts are not our responsability, so although we always try to keep everything backward compatible, sometimes the user itself made so many custom changes on his code that this will turn into a headache when these scripts must be updated to match the latest official changes
it still worth update these scripts to keep using the latest updates (56d) rather than use an old sphere build (56b), but anyway, you can find some official 56b builds here:
https://github.com/Sphereserver/Source/releases
the latest 56b build is from 2013, maybe it could be better than an 56b build from 2009