Are you planning on replacing the primary with something new? If so I'd leave things as-is and just move the settings for now.
If you aren't and you have a model where the second WAN is a combo port that also can be LAN (MX64/67/MX100), I'd strongly consider moving it as I've occasionally seen these models get configs screwed up during a power outage or similar scenario and lose that WAN designation for the secondary port, and recovering becomes more work.
If it's a model where the secondary WAN is always a WAN it's a bit more of a personal preference thing. As far as I know, there is no technical reason to make the change.
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