Lux Flyer

No, I think you are understanding it correctly. RN is not a valid booking code to apply a new MUA or plus points towards, and so your example would not be able to apply a MUA to Polaris.

As jsloan mentioned, you can pay the MUA upgrade cost to go to Polaris and waitlist for both Polaris and Premium Plus from your economy fare, and then you would receive the refund back after the fact if you only clear to premium plus. This would "allow" you to be upgraded from RN, but it has nothing to do with you being in RN, it is the fact your MUA to Polaris was already paid for based on what was due from being on a T fare. You could have gone to PZ from RN or from T (had you never cleared to RN), because you already paid for a MUA to go all the way to PZ.

The situation that runs into the problem is if you initially paid for a MUA to go to only RN from T. In that case, you could not later apply a new MUA to go from RN to PZ. What would need to happen is the existing MUA would need to be cancelled/refunded, placing you back in your original T fare, and then a MUA to Polaris would need to be requested (with Premium Plus also waitlisted if desired). A good agent should be able to save you the RN space you had previously secured, but technically there is no policy that suggests they are supposed to do that, and would probably relying on the agent knowing how to do it and to grant an exception to override the automated process which they are instructed to let handle all this. Moral of the story: There is a risk of losing your RN space if you initlaly only MUA'd to RN but later decide you want to list all the way to Polaris, if you ever think you want your going to want the MUA for Polaris, just apply it to go to Polaris from the get go. Or if you're going to decide to do it later, make sure PZ is already available when you go to update the MUA so it doesn't matter if they lose the RN space.