When you migrate to GA4, you will still have access to all your data from Universal Analytics for six months (after July 1 2023). However, it's important to note that when you compare GA4 vs GA3, the former uses a different data model than Universal Analytics, so you won't be able to merge your data together. And if you want to hold onto your Universal Analytics data indefinitely, then it’s important that you export your historical UA reports during the six-month period, or you risk losing your data.