Consequences of adding domains prior to updating DNS

I'm migrating a number of domains from a self-hosted mail server over to Fastmail. On two of the domains a few users have a good number of aliases.

To avoid dropped email to aliases during the migration, can I do the following?

1. add all domains to Fastmail without updating the MX records on my DNS hosting provider. Mail continues to be delivered to my old setup.
2. create the aliases. This may take a few hours or days.
3. then update MX records to point to Fastmail.

Are there any problems with configuring Fastmail with the domains, without first updating the DNS hosting provider?