Not positive on these answers, but you might reach out to Support about migrating the landing pages. I know they can do "instance copies" but I'm not sure if they have the level of granular control to copy over just landing pages. It then also bears the problem: what if the landing pages are local assets of programs - would they have to copy over the programs too?
However, you end up copying over pages, here's what *you* would need to do in Marketo:
1. You can set the same URL in your new Marketo instance, but you may have to set it manually in the new instance for each page, using the URL tools: Change the Landing Page URL - Marketo Docs - Product Docs. Whatever comes after the .com/ needs to be the same in the new instance as in the old.
2. When you're confident you're ready to switch, your IT team can update the CNAME on your domain to point to your new instance instead of your old one. Just remember it can take 24-48 hours for DNS records to update globally, so you'll want to do this at least a few days before your old instance gets shut off! Customize Your Landing Page URLs with a CNAME - Marketo Docs - Product Docs
3. Unless Support can migrate them for you, you'll have to rebuild the pages in your new instance.
My advice would be to audit your landing pages and gather a list of which ones you actually really need to migrate/rebuild. You can also set up redirect rules in your new instance for the ones that aren't worth rebuilding.