* Currently there is no support for "Auto Upgrade" of new unit that joins the existing stack.
* If a new unit is running a different version than what stack is running, then it won't become the 'active' member of the stack, instead it goes to 'dormant' state, with flag set as:
[V] - Version Mismatch
* With this situation, upgrading/downgrading only that unit/member becomes challenge, as it has to be removed from that Stack and then upgrade/downgrade to same version as that of stack.
* Hence, it is always best practice to load the same version image on new unit(s) as that of Stack, well before 'adding' them to the stack.
Now, since you wish to upgrade the stack as well, you can do so.
But, as mentioned above, the new unit also should be loaded with same image as that of stack (either before or after stack upgrade)
Rgds,
-Vinay