This is very difficult to do. Typically, schools require that you have a compelling personal reason, such as a spouse that is relocating to a different city. (Wanting to transfer because you cannot find researchers to work with at your current institution is not an acceptable reason. Basically, the reason has to involve either a wedding ring or a death certificiate.) Even with that, the school you seek to transfer to has little incentive to take you: they have to dig up funding somewhere to support you. In the past, the NIH would give an additional training grant slot to a program accepting a transfer student. The diminishment of the NIH training budget over the past decade has caused that practice to be discontinued.
