自宅作業用PCのFreeBSDを6.0から6.1-RELEASEに更新した． src/Makefileにあるように，

# For individuals wanting to upgrade their sources (even if only a
# delta of a few days):
#
#  1.  `cd /usr/src'       (or to the directory containing your source tree).
#  2.  `make buildworld'
#  3.  `make buildkernel KERNCONF=YOUR_KERNEL_HERE'     (default is GENERIC).
#  4.  `make installkernel KERNCONF=YOUR_KERNEL_HERE'   (default is GENERIC).
#  5.  `reboot'        (in single user mode: boot -s from the loader prompt).
#  6.  `mergemaster -p'
#  7.  `make installworld'
#  8.  `make delete-old'
#  9.  `mergemaster'
# 10.  `reboot'
# 11.  `make delete-old-libs' (in case no 3rd party program uses them anymore)

という手順1で問題なし…

  1. 知らない間に，make delete-oldとmake delete-old-libsが追加されていた． ↩︎