Saving Project 2007 Project in other formats – what is lost?

If you save a Microsoft Office Project 2007 project to other older formats such as Microsoft Project 2000–2003, you
can lose data in areas where Office Project 2007 contains enhanced functionality. In particular;

Budget information	Information contained in the new budget (budget: The estimated cost of a project that you establish in Project with your baseline plan.) fields will be lost.
Deliverables and Dependencies	Information contained in the new, cross-project deliverables (deliverable: A tangible and measurable result, outcome, or item that must be produced to complete a project or part of a project. Typically, the project team and project stakeholders agree on project deliverables before the project begins.) and dependencies fields will be lost. Information contained in task links will not be lost.
Cost resources	Information contained in the new cost resource (cost resources: Resources that don't depend on the amount of work on a task or the duration of a task, such as airfare or lodging.) fields may be changed, converting cost resources into material resources. Information contained in other cost fields will not be lost.
Background cell formatting	Sheet cells that have background formatting applied to them will lose their background formatting. Information in these fields will not be lost.
Assignment owner information	Information contained in the new assignment owner fields will be lost.
Calendar exceptions	New exceptions (exception: An instance in which a task or resource calendar differs from the project calendar. An example is a resource that must work on Saturdays, but for which the project calendar does not include working time to cover these instances.) that have been added to project and resource calendar functionality (such as monthly exceptions, yearly exceptions, and those with arbitrary dates) will be lost.
New fields and custom fields	Information within new fields, or any formulas relying on new fields, will be lost. New options set on custom fields created in Office Project 2007 will be lost, such as the new roll-down setting for calculation of assignment rows. Task assignment custom field values will also be lost.
Enterprise custom fields	Any enterprise custom fields created within Office Project 2007 will be lost.

Something to bear in mind when downgrading your project format.