It is indeed very true that one can use an APS-C or DX (for Nikon) and EFS (for Canon) format lens on a FX or full-frame DSLR. For old timers like ourselves started in the film days, all lens produced were "FX" by definition. Also like the majority of us entering the digital era through the APS-C format, the single "inconvenience" is the crop factor, ie the conversion factor for Canon at 1.6x and Nikon at 1.5x as well as the loss of the wide angle perspective. Conversely, for bird photographers, there is the welcoming gain in the telephoto perspective. However, with the entry to FX format DSLR, a 50mm lens is once again a 50mm lens :|
To under the detail of the APS-C format that resulted in the crop factor issue, the following article is very helpful to illuminate the various facets of this interesting phenomenon:
http://en.wikipedia.org/wiki/Crop_factorTo understand further the nature of the APS-C or DX format lens, the following short reference may be helpful with future investment decisions:
http://www.kenrockwell.com/nikon/dx-lenses.htmGood birding and photography.