Thank you for posting this in Microsoft Q&A.
Yes, there are some additional features that you get with E5 license. For example, with E5 you get additional feature like, Identity protection, Information protection, Threat protection etc.
These additional features would be available only to users to whom you assign E5 license.
There are no restrictions or consequesces if you use these security features without upgrading E3 to E5.
To know more about the feature you get with Microsoft E5 you can check below article,
https://www.microsoft.com/en-us/microsoft-365/enterprise/microsoft365-plans-and-pricing
Let me know if you have any further questions.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.