I´d like to change MX from On-Prem to Office 365 even there are only 10 mailboxes and 4xx On-Prem
If you change your MX record to point to the Exchange Online Protection service in Office 365: This is the recommended configuration for hybrid deployments. All messages sent to any recipient in either organization will be routed through the Exchange Online organization first. A message addressed to a recipient that's located in your on-premises organization will be routed first through your Exchange Online organization and then delivered to the recipient in your on-premises organization. This route is recommended if you have more recipients in your Exchange Online organization than in your on-premises organization. This configuration option is required for Exchange Online Protection to provide scanning and blocking for spam.
The main reason is.. Using EOP instead of old On-Prem AntiSpam
Should I also change autodiscover record?