No, spindritf has a point. If a MTA is configured to consider a domain as "local" it will route emails internally instead of doing MX lookups. It's likely more complex in this case, but essentially it can mean that any emails routed through Google's systems (Gmail and Google Apps for Domains) will keep bouncing as long as the domain is configured, even if you've changed MX records to a different MTA.
Based on the quality of other Google products, I think it's safer to assume the inverse: Google's mail servers likely know about a domain's MX records and adjust accordingly without account configuration or external intervention.
Maintaining a system as large as the one they do without this feature would seriously suck for the mail admin - it really has nothing to do with the customer at that point. I wouldn't want to work that job if it did because of a) all the angry people it affected and b) the overhead in my job introduced by a bad programmer.
This is a fact seen in practice. You can check it if you want. Just create Google Apps account and configure mail without setting any MX records in your DNS. You will be able to send and receive emails to and from that account and regular Gmail account.
Not a complete counter example, but on a domain I run with external mail, gmail enabled but MX never setup, sending a email with google apps gmail, from me@mydomain to you@mydomain sends the email to the external provider
I believe spindritf is indicating that google doesn't use the "open internet" for gmail <-> gapps, it's all routed internally. If they find your domain registered within their system they skip the lookup.
Obviously any email sent in the interim will be lost, but any email delivered after the MX record change should just be sent to the new provider.