Custom A Records

I currently host a website with a subdomain that hosts phplist and local municipal mx records which I wish to maintain at its current address.

I created custom A records to point at a host for a touchy CMS I am involved with. I have created the domain and mirror on DreamHost and have recreated the CMS in an updated version on DreamHost.

I want to now ditch the custom A records and create new records that would point to the DreamHost servers but do NOT want any interruption in the MX records for that domain. How do I handle this on DreamHosts MX records so that continuity is maintained?