As mentioned by @abhinavbalooni, they are just different approaches to data collection. You may think about the existing AppMeasurement and new WebSDK tracking is taking mass transit or driving to the office.
Most of the analytics data should not be impacted, and calculations happening on the AA side, such as visit definition, should not impacted. However, there may be some minor differences you need to be aware of by referring to Variables, functions, methods, and plug-ins overview | Adobe Analytics which documented AppMeasurement and WebSDK approach of data collection.
Some features may be missing in the WebSDK, such as offline tracking and also Activity Map which was only added into WebSDK on Mar-2023. some features may be implemented differently, such as there is no dynamic in WebSDK but need to implement that in Datatream to map incoming data to different AA dimension