domain name system - Is it possible to change MX records without downtime/returning/dropping messages?

We're migrating e-mail hosting from SERVER-A (on HOSTING_CO-A) to SERVER-B (on HOSTING_CO-B).

SERVER-A will continue to be functional until the transition is complete and HOSTING_CO-A has their own DNS server since our records currently show something that looks like:

mx1.hosting_co-a.example

ns1.hosting_co-a.example

and HOSTING-CO-B presumably has their own DNS servers as well (it is Google/GSuite).

I've read [1] that DNS changes may take up to 72 hours.

What would be the worst case scenario for what happens to an e-mail sent during this period?

1. Would they either go to SERVER-A or SERVER-B (that is, would e-mail providers using non-updated DNS records be able to get through to SERVER-A or would SERVER-A reject or forward it based on its updated MX-record)?




2. Would they be 'dropped' (that is neither the sender nor recipient will be aware that the message didn't go through)?




3. Would the sender be notified that the message was not delivered?

We are switching over various domains/accounts, for some domains we'd rather messages be dropped than returned, and for others, we'd rather returned than dropped - if this is even something that can be controlled.

[1] https://support.google.com/a/answer/45679?hl=en Also points out reducing TTL for records in advance (would this mean a maximum downtime of your TTL?)