Migrate from Web IDE to BAS on-premise

We are trying to see if moving to BAS from WebIDE is possible (or recommended) for on-premise scenarios. All the SAP documents and blogs I've seen on the subject only talk about cloud, there is literally nothing about on-premise environments.

My questions are:

1. Do we need to upgrade the on-premise runtime environment from Neo to Cloud Foundry? If so, how do we do that? If not, how do we deploy Node.js apps in BAS to a Neo on-premise runtime?

2. Can we keep some apps in Web IDE, and migrate some apps to BAS to test it out? or is it all-or-nothing?

3. Is it common practice after migration to BAS to perform complete UAT of the deployed apps? The logic hasn't changed, just the file structure. What are best practices? (UAT of all our apps will require a lot of time/effort from the client business groups)

Thanks in advance.