If you have not started taking the exams already, I would highly recommend you go straight for the OCP8i, that way you only have 5 tests to write to become OCP8i rather than 6 if you first take the OCP8 exams. I did OCP8, so now I have to take another 6th exam to become OCP8i.
To anwser your question, why not just get OCP8i materials? I do know a friend who inadvertently took OCP8i for the network exam while studying in preparation for OCP8 and she passed, but only just. So you better get 100% for all OCP8 questions that show up in an OCP8i test, otherwise you chances of passing are dramatically reduced.