Upgrading solutions

When we upgrade our Dynamics CRM organization to Dynamics 365 CE, all of the customizations are also upgraded automatically. However, we still need to work on our customizations to adjust them based on the new Dynamics 365 CE interface. If anything unsupported is done, we need to remove it to make sure everything is working with Dynamics 365 CE correctly. We may need to change our navigation based on Dynamics 365 CE. Depending on which version we are upgrading from, we need to work on the customization. Before Dynamics 365 CE, we used to develop custom solutions for features that were not present in Dynamics CRM—for example, multi-selection dropdowns. This feature was not available in earlier versions, but now, we have this feature out of the box. We should always try to use out-of-the-box features of Dynamics 365 CE instead of writing any custom solutions...