Should be a straight forward process to migrate from one batch of external IPs to your new allocation. Note your existing settings on your MX, NAT rules etc then amend to the new.
Darren OConnor | doconnor@resalire.co.uk
https://www.linkedin.com/in/darrenoconnor/
I'm not an employee of Cisco/Meraki. My posts are based on Meraki best practice and what has worked for me in the field.