Here's what i have:
wan: 64.8.x.x (public)
lan: 8.224.x.x DHCP (public)
my network (before mikrotik) is setup with static 172.20.x.x addresses as well as a few static 64.8.x.x public addresses.

I would like to keep some of the 172.20.x.x addresses as well as the 64.8.x.x addresses in place.
How do i accomplish this?

Thanks,
Ron

for clarification:

My network's set up so that all my CPEs have static 172.20.x.x addresses. this equipment is acting as a bridge for the customer. the customer then obtains a public ip (64.8.x.x) from our dhcp server. a few of those customers have static public ips (64.8.x.x).
the 64.8.x.x addresses are needed elsewhere on the network and are being replaced by the new block of 8.224.x.x (coming from the my new MT box).
I still want to be able to access the equipment that has the 172.20.x.x and 64.8.x.x addresses once i plug it all into my MT box along with having them be able to access the internet.

Anybody??

If the new network is exclusive to the original one(meaning they won't see eachother, you could make a private network on the LAN of the new MikroTik, and simply NAT their devices through the new IPs, though I think going ahead and making the static IP changes will be worth your time in the long run.