Did one or the other (i.e., Office 2019 or Office 365) come with your system and then you installed the other application?
And you wish to uninstall Office 365 - correct?
You should be fine doing so but take precautions just to be safe.
Check your file type associations: If possible determine which one (Office 2019 or Office 365 is the default application for .docx, .xlsx, etc..
If Office 2019 is opening those file types then Office 2019 is the default application.
In any case do backup all data and verify that the backups are indeed accessible and readable.
The uninstall process can be iffy and may go maverick on its' own merits.
Be ready to reinstall Office 2019 if that becomes necessary.