"Field Replaceable Units are those parts expected to be replaced by a Lenovo authorized service technician. Only Lenovo authorized service technicians may perform these steps under warranty.[...]"
So I need to contact a Lenovo service. I'm hope it will not cost a lot. O_o
If your machine is still covered by a warranty, this would be your best bet anyway, although i doubt that they will do anything that is not related to the original machine's part delivery. When i had to send in a T61 for mainboard replacement under warranty due to the infamous nVIDIA bug, i insisted to rather put in an Intel based mainboard instead of a nVIDA based one, but which they openly refused based on these very grounds. Assuming you succeed, supposedly Lenovo will be happy enough to charge you a large enough amount of money which would have sufficed to buy a good used replacement T400.
For this reason alone, lots of people (me involved) prefer to do all the FRU replacements themselves, unless a still valid warranty would be put to risk. Thinkpads were build for being serviceable, and thus can be easily dis- and reassembled given proper documentation, which can be had at the aforementioned lenovoservicetraining website.