Thanks for the input... now a little follow-up...
First, what IS the TTL value? It'll be helpful for me to know this whether it's possible for me to change it or NOT...
Regarding the switchover mechanism... Actually, my PREFERENCE is to do it a bit differently than your suggestion.
Google would call my desired approach a "split delivery" approach with Google as the primary server:
(Usually, I'd want to change TTL values to facilitate fast propagation of changes.)
Set a "subdomain" like "temp.mydomain.com" at DreamHost. This will point to the legacy Exchange servers.
[Any tips about setting up a "subdomain" on DreamHost?]
In Google, (AFTER I believe the changes in step 1 have reliably propagated across the internet) establish "forwarding" rules that will forward all mail arriving at Google to the "temp.mydomain.com" subdomain
Switch DreamHost to send all mail to Google
(Usually, after testing of this setup, I'd change TTL values back to something providing "normal" propagation timing of DreamHost records.
After propagation of the revised DreamHost info around the internet, this approach gives me user-by-user control of my switchover. Whenever I'm ready to move "john smith", I just remove the Google forwarding rule for that person. I will NOT have to go any where else to make that change. Then I can transfer their historic email from Exchange to Google. This will provide more control of which users are switched, which is desirable. And I'd prefer to be working in Google rather than in Exchange.
It would still be useful to know the TTL value in effect with DreamHost records... I'd like to KNOW when changes like my steps 1 and 3 above will be RELIABLE and CONSISTENT across the internet.