Another thing to keep in mind is that if you are upgrading customization content, you need the new versions of your customization tools, if you use these, of course:
a 64 version of DOSLIB ( DOSLib18x64.arx ) at http://download.rhino3d.com/download.asp?id=doslib
a 64 version of ObjectDCL ( ObjectDCL-ARX64-7.2.0-Setup.msi ) at http://www.objectdcl.com/en/arx.html
a 64 enabler of VBA ( AutoCAD VBA module 64-bit ) at http://usa.autodesk.com/adsk/servlet/item?siteID=123112&id=12715668&linkID=9240618
Also, after installing the VBA enabler, the old VBA routines have to be checked one by one. Most of them will work, but some lines of code might need to be changed. Some of them might cause AutoCAD to have a very slow performance while saving. It is recommended to migrate these routines to the new VSTA programming tools.
The very old, good, and simple AutoLisp will still work, as always.