Beamer: Change template in the middle of the presentation

Hello everybody out there using TeX,

After having created several presentations with the LaTeX Beamer class, I'm wondering whether it is possible to change any template after the beginning of a presentation.
In my special case, I would like to change the symbol in front of bibliography items:
However, the change to beamertemplatebookitems is ignored. Could anyone point me towards a way to find the reason and a possible fix for this behaviour?