So the way to fix this which took me an annoyingly long time to find is to use the derived cast method described here. Now the definition is this:
internal::cast_return_type<Derived,const CwiseUnaryOp<internal::scalar_cast_op<typenameinternal::traits<Derived>::Scalar, NewType>, const Derived> >::type cast() const
Which Ill admit, phased me a little. But turns out it is pretty easy (and the only explanation I could find was in the Eigen 2.0 doc which was frustrating). All you need to do is this:
m2 = m1.cast<float>();
Problem solved.