Will I experience downtime if I switch CSP partners?

No — switching the partner-of-record is a change to billing and administrative relationships, not to your actual deployed services. Your Azure resources, Microsoft 365 mailboxes, and other services continue running throughout a partner transition without any expected service interruption.

The main things to plan carefully are the billing cutover timing (to avoid any gap or double-billing) and admin access handoff, which a well-organized transition between partners should handle smoothly.