Claim the new MX serials into your org Disable HA which removes the spare from the network Enable HA and select the new spare serial Bring it online, let if update config and firmware and settle Force HA switchover from primary to spare using the swap button Disable HA again which would remove the old primary serial from the network Enable HA and choose the serial for what will be the new primary MX Bring it online, let if update config and firmware and settle Last, force HA swap again Enabling HA does cause a soft reset on both MXs and will interrupt traffic. So, you'll still want to do this all in a maintenance window.
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