What functionality is available in the version you are upgrading to versus what is in the scope of the upgrade project is the issue that needs to be decided.

In my experience, delivering the current functionality available to the business is the first step towards the new platform. No one would like to go backward on the features they already had. However, you should negotiate for not implementing brand new features as part of the upgrade project itself. These new features can come as a next step after the upgrade, even if these features are available out-of-the-box in the new version of Dynamics 365 for Finance and Operations. You need to consider the time needed for the implementation of such features and the impact to the overall timeline of the upgrade. I would rather run parallel projects to implement several new features once I have moved to the latest version, than delay the upgrade itself. This approach allows you to divide the scope into smaller projects for the new features and is easier to manage as well.