If your client insists that you work on his file, he could send you an IDML export. I also forgot to point out a feasible (but not really recommended) way of working with newer documents. How could the programmers of CS work with footnotes (CS2 and up), or have foreseen GREP styles in CS4, or column spans in CS5? If it was possible to open a CS4 document into CS3, what should happen with the features that Are Not Available in the older version? What you meant is " forwards compatible", and that's more than just a minor technical problem. (The Paragraph Composer, for example, was rewritten for CS3, so editing a pre-CS4 file into a newer version *will* make text re-flow.)
InDesign is backwards compatible - up to a point, at least. With software upgrades being released almost annually that are not "backwards compatible".
#ADOBE INDESIGN CS4 UPGRADE UPGRADE#
That's a good point, if this one client is the sole reason to upgrade to CS5. How can I justify paying Adobe another $599 for an upgrade on a job that will only net me about $200 ? Hell, I just paid Adobe $899 last year to get CS4. Apparently, they expected everyone to upgrade immediately to any new version of ID as soon as it came out.
The second reason is more mundane: the Adobe programmers argue that if InDesign cannot open a document, then logically it must be because of someone other's plug-in. (Note that many of them have no, or a very small fee, for upgrading across versions.) It's the plug-in writer's responsibility to stay up-to-date. This warning applies to documents containing stuff that has been created with thrid-party plug-ins, and to be able to open it you need the correct version of that plug-in.
First off, Adobe isn't the one writing custom plug-ins (I think they don't even do that at all). What I CAN'T understand is how can I (or anyone else) "upgrade my plug-ins" when Adobe doesn't make any plug-ins available on their web site?