Yes, I realized that. My answer is just my thinking on which is likely to be the easiest way through the situation for you.
If you are given promotion right away, you still have to notify INZ of the change.
If you apply for post-study employer assisted visa, with WHICHEVER job title, there are MANY fewer formalities than for SMC, and you can quickly get a visa that keeps you legally working in NZ. This would give you time to deal with all the paperwork for SMC with no rush.
You are allowed to put in your EOI at any time - you don't have to wait till near the end of whatever visa you are on.
If you got your promotion to retail manager really soon - say, next week - and decided to apply for residence under SMC, it is unlikely to come through in 'a few months', the time you say it is till your present visa expires. You would still have to put in for a new work visa.
So whatever happens, you are likely to have to apply for two visas - the employer-assisted work visa AND residence under SMC. The only question is, which way round. People who go for SMC first tend to get tense, thinking, 'Is it going to come through in time before my old visa expires? Is it? Is it? Is it? Oh, it isn't. Now for my work visa application - is it going to come through in time?' and so on. People who get a work visa, with two years legal living and working ahead of them, may still get A BIT tense waiting for INZ to reply about their other application for SMC residence, but at least they haven't got the worry of an expiring visa which might leave them not allowed to work while it's sorted out.