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With regards strictly to Marketo, this will have no impact at all and there is nothing to do prior or after the change.
Also, you said "marketo landing pages." Changing the IP of your main website has NO impact to your Marketo pages, which are hosted on Marketo at a separate IP that has a pointer from your DNS record.
As Greg said, there is no action to take.
The only caution I would have is if someone created an API between your site and Marketo and for some reason had an IP hard coded or had other things that might get messed up from a security standpoint.