Good news: after the upgrade to Wheezy the plug rebooted with the new SD card without me having to push the reset button
Bad news: as with my first attempt to upgrade, smbd does not want to start. I got the below syslog message.
Thus I put back the "Squeeze" card for the moment, leaving the "Wheezy" for trouble shooting with my spare plug.
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.289649, 0] lib/fault.c:47(fault_report)
May 9 17:06:45 Share smbd[2430]: ===============================================================
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.295058, 0] lib/fault.c:48(fault_report)
May 9 17:06:45 Share smbd[2430]: INTERNAL ERROR: Signal 7 in pid 2430 (3.6.6)
May 9 17:06:45 Share smbd[2430]: Please read the Trouble-Shooting section of the Samba3-HOWTO
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.296434, 0] lib/fault.c:50(fault_report)
May 9 17:06:45 Share smbd[2430]:
May 9 17:06:45 Share smbd[2430]: From: http://www.samba.org/samba/docs/Samba3-HOWTO.pdf
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.297702, 0] lib/fault.c:51(fault_report)
May 9 17:06:45 Share smbd[2430]: ===============================================================
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.298565, 0] lib/util.c:1117(smb_panic)
May 9 17:06:45 Share smbd[2430]: PANIC (pid 2430): internal error
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.300102, 0] lib/util.c:1221(log_stack_trace)
May 9 17:06:45 Share smbd[2430]: BACKTRACE: 0 stack frames:
May 9 17:06:45 Share smbd[2430]: [2013/05/09 17:06:45.302107, 0] lib/util.c:1122(smb_panic)
May 9 17:06:45 Share smbd[2430]: smb_panic(): calling panic action [/usr/share/samba/panic-action 2430]
May 9 17:06:53 Share smbd[2430]: [2013/05/09 17:06:53.908783, 0] lib/util.c:1130(smb_panic)
May 9 17:06:53 Share smbd[2430]: smb_panic(): action returned status 0
May 9 17:06:53 Share smbd[2430]: [2013/05/09 17:06:53.909335, 0] lib/fault.c:372(dump_core)
May 9 17:06:53 Share smbd[2430]: dumping core in /var/log/samba/cores/smbd
May 9 17:06:53 Share smbd[2430]:
