Mixing Fowlers patterns
I don't think that there is inherently anything "wrong" with having multiple patterns like this in a single application, especially during a refactoring exercise. I would strongly suggest documenting the patterns that you are changing from and to and the reason for the modification so that future developers understand what is happening.

I would make one substantial modification to your proposed implementation, however: define Enrol at the Student level as an Overridable method. 

If there is no default implementation for all students, change Overridable to MustOverride. This will in turn force Student to be declared as MustInherit, but this is ok if you can only call Enrol on instances of Undergraduate or Graduate.