Revit does not save backward. Said another way, you cannot save from "this" version to a previous version's format. It never has..maybe never will...but never say never?
One common concern or comment is based on working with consultants, "What if my consultant is using the previous version of Revit? What then?"
The only thing preventing a consultant that is currently using 2008 from collaborating with "you" using 2009 is their subscription status (that and a willingness to install it). They can work with you using 2009 even if all their other projects are in 2008 as long as their subscription is in force. Technically all they'd need is a single seat installed for 2009 (for each user) and a valid subscription for that seat, even if they didn't have a valid subscription for their remaining seats.
(I should clarify that I'm not a reseller and you really need to make sure that your firm is in compliance with the Autodesk End User License Agreement(s), EULA for your products. You really don't want a visit from the piracy folks. Please don't make such decisions on the basis of my post alone! Agreed??)
This means for "your" project the consultant would need to renew their subscription for each necessary seat to get the current version up and running apart from the "EyeTee" department exerting some effort. How much cash is required depends on how many people will need to work on the project and how long ago they stopped subscribing.
Note: In effect a Revit firm that abandons subscription is truly deciding NOT to collaborate... Autodesk definitely has us coming and going on this point.
A "team" of firms using Revit really MUST agree WHEN to upgrade to another version. One part of the team arbitrarily deciding to do so will unfairly burden the others with the timing of an upgrade. It is not necessarily a hard thing to do, normally, but it is very likely to be inconvenient depending how soon the next deadline is.
No backward compatibility has been and continues to be motivated by a greater urgency for moving forward than working out the complexity of supporting firms that don't "want" to upgrade. Anyone trying to truly collaborate with all disciplines right now will agree there is MUCH that needs to be done for every discipline to allow greater and tighter integration of our data.
Some might say that Autodesk "likes" or "enjoys" this situation, being necessary to maintain subscription, and perhaps there is some truth in that. The reality is that in order to progress they must be able to focus forward. If we demand that each release must save back to earlier releases then a considerable force must be applied to make that possible.
Consider that features added in 2009 might not be possible to create in earlier releases at all. A Swept Blend did not exist in an earlier release, what shall it become when saved to an earlier release? Just delete it? Recreate it unfaithfully because the geometry can't be represented? Create an ACIS "exported" and then "imported" instance in the model that can't ever be altered? I imagine that whatever elements are affected by such conditions that the resulting form(s) will not have the same fidelity as the original which calls into question the logic of doing so at all.
An enormous amount of effort would be necessary to permit this when the much "easier" (for Autodesk perhaps) solution is for customers to use the same release. Compare the few hundred dollars for a subscription per seat required of the customer as opposed to the kind effort required to make this possible in any fashion and you'll see it isn't a very motivating project for them.
Seth Godin wrote about Love and Annoying the other day. He wrote this:
...The goal is to create a product that people love. If people love it, they'll forgive a lot. They'll talk about it. They'll promote it. They'll come back. They'll be less price sensitive. They'll bring their friends. They'll work with you to make it better.
If you can't do that, though, perhaps you can make your service or product less annoying...
Read more...he goes on to say that Apple makes products people love as an example and that an Airline ought to work hard to make air travel less annoying. Seems to me that this issue and Revit as a product is on that "fence" now. People have been and continue to be passionate about Revit and willing to forgive its quirkiness. Increasingly many users however are more interested in it becoming less annoying.
Seth closes with this:
...Put a sign on your office door, or send a memo to the team. It should say either, "Everything we do needs to make our product less annoying" or "Everything we do should be idiosyncratic and engage people and invite them to fall in love with us. That's not easy, which is why it's worth it." Can't have both. Must do one...
It might seem that I am apologizing for something that Autodesk should just "do". Perhaps. At this time I am much more interested in Autodesk spending time and resources to add functionality so that firms can collaborate better. Every penny spent to support backward motion is "negative equity" and as we know that sort of equity has been contributing to the current state of our economy in a fairly significant way.