Update OS on Force10 MXL stack

I need to update a stack of 6  dell mxl 10/40gb switches but the last time I updated the stack I did not have any production on the stack and just reloaded the stack after it was done updating. At this time I can't just reload the stack because it is running ISCSI traffic on a lot of vmware hosts.

Can I update the stack and reload one switch at a time and will the updated switches join the stack with different OS versions?

Need to go from 9.14(1.9P4) to 9.14.1.13