After installing Microsoft Office Compatibility Pack, my Office XP can open Office 2007 format files.
Even though I cannot see the internal operations, conversion between DOC and DOCX files should be easy because one of the format is XML based.
What I am interested at is that when designing Office XP, the guys left an interface to add new file format support. Thus, five years later, we can enjoy the support for new formats.
Yes, a delicate design could benefit you long after.