Unfortunately you can not update the SDM template (commonly done to enable IPv6) on a 3750 switch stack without downtime.  The slave switches will automatically use the SDM template that the master is running.  SDM templates take effect on a reboot so here’s what happens when you try to do it on a stack:

1.  Set SDM Template so on reboot switch stack members are supposed to come up with new SDM template.

2.  Reboot master switch.

3.  Slave switch now becomes the master and is running the OLD SDM template.

4.  Old master boots up and comes back into the stack as a slave.  This slave utilizes the SDM template that is running on the master which is the OLD template.

5.  Repeat infinitely with same results.

3750stack# conf t

3750stack(config)# sdm prefer dual-ipv4-and-ipv6

3750stack(config)# end

3750stack# reload

The reboot tends to take about 5 minutes, which is an eternity when you have something high-availability enough that you would put it on a redundant switch stack.