M4300 creating a stack with a new member

> I have set the priority to 15 on the current switch 

That's a good thing to do it makes the stack behavior more predictable not just during expansion by new stack members.

> and set the stack ports.

Just to be certain, check the current operation mode using the "show stack-port" comand. Switching the mode requires a reboot to make it active.

> When I power the units off and back on will all the configuration of the original stand alone unit be retained and resume function? 

Here are some things you can consider and check, but you should also consider the M4300 Software Administration Manual (Chaper 21).

• I'd always first get a config backup from the current config (for the "just in case") of the switch in production.
• Even though the current switch doesn't need rebooting, consider doing the integration outside of lecture hours (again: "for the just in case")
• use the "member" (Stack Global Config) to add the correct type of M4300 model to the current stack master and the preferred stack unit ID of the new switch (this can be done in advance with no interruption)
• Then it does also help to get the new [1] stack member to the same firmware (and in the same boot bank) as the current stack master.
• So long as the operational / running mode of the designated stack ports is set to stack and not ethernet, the current switch can remain running "as is."

You can then connect the designated / configure stack ports between the current and new switch, then power up the new stack member.

What should happen then, is that the new switch comes up, realizes there is a current stack master, selects its stack unit ID you have previously and joins the stack and copies the config from the master. Use the 'show switch' command to verify that the new stack member has properly joined the stack.

[1] While stack firmware sync does work, it adds another step to wait for the master to sync and integrate the new stack member.