Historic data: The data in your old properties will continue to be available in a read-only state, but this data cannot be imported into your new GA4 properties. Eventually, this historic data will not be readable, which is why you should at least get a GA4 property setup asap to run concurrently with your UA property.
Tagging update: Google Analytics 4 tags collect data in a different way than Universal Analytics. As such, you'll need to set up new tags with the latest version of Google Analytics. Proper tagging ensures that you have visibility on all your data and that your website communicates properly with third-party properties.
Team training: Google Analytics 4 is significantly different from Universal Analytics. Most teams will benefit from carving out time to familiarize themselves with the new tool. Even if you do not actively manage Google Analytics 4, there is new terminology that will be important to understand.
Media platform integrations: Each Google Analytics property is unique in that it has filters, audiences, and goals specific to a particular business. The goals and audiences that you have created in Universal Analytics will need to be recreated in GA4 and shared with your media buying platforms.
It's simply better: Google has had a decade to figure out what the new and improved version of Analytics will look like. GA4’s improved data model provides a more accurate and complete picture of how users are interacting with your website or app. In short, the data is more predictive, useful, and accurate. Don't worry, GA4 is still free.