Hi @amrish_popat ,
Thanks for kicking off this discussion!
It sounds like you are planning on running both Web SDK and AppMeasurement.js simultaneously for a period of time so you can compare the two Analytics implementations side by side. That is outside the scope of this Target migration tutorial, but there are other migration materials we are working on which I am very close to. We have other customers using this approach to help ensure a smooth migration of their Analytics implementation, and I think it's a really good idea. It will result in extra Analytics hits, but you can limit this by simply shutting off the Web SDK sendEvents when you have the amount of data you want.
When it comes to migrating Analytics to Web SDK now and Target later, it hasn't been extensively tested. You can do what you want, but Adobe wouldn't be able to help you if you run into issues, hence "not supported". If you use Analytics data for your Target reporting (A4T) this will definitely break. I can tell you from my personal experience as the implementer of Adobe Target on the Experience League docs and tutorials, we are using Web SDK for Analytics and at.js for Adobe Target and I have not experienced any issues. I am not using A4T. Also, the Web SDK Analytics implementation was already in place before Target was implemented, which is slightly different from your scenario.
"Does that stop us from going Live?" You can do what you want, but you will be limited in your support from Adobe if you encounter issues.
"Do we have to disable AppMeasurement before WebSDK GO Live?" There are other customers who have used this approach, so I think you should be fine with this, especially since this is for a limited time and you are using separate report suites.
"Will it impact AEP data ingestion?" You should be careful if you are currently using the Analytics source connector to ingest data from your AppMeasurement report suite into Platform. If you have two Analytics implementations feeding into Platform and compromising your data integrity.
"Will it impact Target activities/operations in Live" It will definitely, impact A4T. The rest, I can't tell you for sure. Again, this is "unsupported" territory.