As indicated by dialabrain you only need to upgrade Office if you will be switching to the new macOS Catalina when it is released.
If you do decide to upgrade then if you currently have an Office365 subscription this subscription always gives you the ability to upgrade to the latest version as part of that monthly subscription cost.
If you have the older style one-off purchase of an Office license then you either have to buy an Office365 monthly subscription or if it is still available. (Even if a perpetual Office license is still available today as far as I am aware Microsoft intend to discontinue this approach very soon.)
Yes the newer version will look different.