I have 2811 with PVDM2-32 with c2800nm-ipvoice-mz.123-14.T4.bin.
I have no problem with add or reduce max session command.
If I understand correctly, PVDM2-64 has 4 DSP.
Each DSP supports 4 max session transcoder OR 2 max session conference.
for instance, you configure 1 DSP for 4 transcoder session, then you will have 3 DSP for 6 conference session.
Thanks