You can Add a new Relation Model, in the Catalog Explorer, to use the ODP.NET driver.
And then you can start changing the mapping of each entity to the newly added relation model. (This can be done by editing each entity and using the Field Maping tab to create a new mapping instead of the old one).
This can be tricky since you may find some fields .NET Types mimatching with the new relation model database types, since ODP.NET & MSOracle sometimes interpret numeric fields differently.
Personally I'd have created a new project from scratch. And keeping the old one as a reference.