How to implement the repository pattern for an app that will change its database from sql to nosql on in couple of years?

I have been reading a lot about repository implementation. I am confused about which way to implement it for a project I am sure would change its data layer methods because of db migration from MS Sql Server to NoSQL in a couple of years.

Imp #1

• Implement Rep layer as a completely separate layer. This will have its own interface and conversion methods here. This rep layer has a dependency on DAL and in turn BLL will have a dependency on this layer

Imp#2

• Implement Repository not as a separate layer but have its interfaces in Business Logic Layer and the methods in DAL.

I am leaning more towards imp #1 since it looks cleaner. However some explanations of experts I have read use imp#2. I think there must be a clear reason to use one over another according to the situation. I would ideally want our switch to a different database be as painless as possible.