Theoretically, you can transplant cages of the same era, as the critical cage attachment dimensions are the same. See attached picture of a Deore XT cage that I grafted onto a 600 Ultegra body. However, you will not necessarily gain all the extra capacity. While you will gain chain wrap capacity, you will almost certainly not gain maximum cog capacity. I didn't. This is because the geometry of the bodies for the long cage derailleurs are different. By using the same body, you not affecting the ratio of downwards to inwards cage travel and therefore the maximum cog capacity will not increase. The only way the graft would cause extra cog capacity is if the cage geomtery were different (i.e. position of the upper pulley relative to the cage pivot), and this is not significantly different, at least in my case. Of course, you can gain the some extra capacity by playing with the B-spring adjustment and wheel position in the dropout dropout, but at the expense of shifting performance. The only reason to graft a long cage is to gain the extra chain wrap capacity to permit installation of wider ratio chainrings.