Sorry I did not mention it earlier because you had stated that you had already started your work in the current version. If you use an older release you will have to recreate your model.
Sorry to Necro the post, but this is actually a significant problem. Simply saying "Talk to your consultant before starting work" is NOT a solution. It simply doesn't always come up ... and to have to switch between different versions of the program between projects can become a hassle.
Not to mention having to load multiple versions of the SAME program onto a computer is just a waste of hard drive space! If other Autodesk programs can down save, why can't Revit?
Look I am an Architect not an employee of Autodesk. I have been using Revit for over six years. When I start a project that will include other team members working in Revit I we figure out which version will be the best for all to work in. My preference is always to use the most current because of the new feature sets and productivity gains. Hard drive space is cheap I don't mind having a couple of versions of Revit.
When new features are added some of them become part of the model itself. If you try to take them out so they are compatible with an earlier release that does not have that feature then the model ends up be dumb downed to a point where it is losing too much information, basically making it useless as bim file. So everyone needs to agree on which version to use period.
Actually the same thing happens with AutoCad every three years when they upgrade the dwg file format. The only difference is that when the file is dumb downed to be compatible with the previous cycles of autocad your are left with just lines. That works fine with autocad because that is it's basic building block. Revit's basic building block is data (information). Once you start losing that your left with garbage.
If this is a problem that comes up several times a year, why doesn't Autodesk do something about it? The obvious reason is money, of course, but Revit isn't the only BIM software out there, and this is a significant issue. I'm not sure how easy it will be to change our office's 8 subscriptions, but this is enough to make me seriously consider it on principle. I find this practice underhanded at best. Congratulations on ruining your reputation with me. I'll make sure I pass it around when our clients ask if they should be upgrading to Revit.
Despite common beleif, it is possible to covert files to an earlier version. You just have to save the file as an IFC and then it can be opened in previos versions. You may have to tweek the placement of objects when you open it in an earlyer version, but it is better than nothing.