Enthuware

I was learning to pass OCJA 7 exam - using many mock exams (Enthuware, Whizlabs and so on.) Getting scores above 80%.

I wanted to write exam next month, but I heard opinion that it is not worth to write OCJA 7 and I sholud try OCJA 8.

Are there many differences between those exams? Is it worth to start learning from scratches for the different certificate? When I get good marks on older one?
What is worth drawing attention to?

There are several differences between 7 and 8 but basic concepts required to clear any of them are same. So your effort will not go waste even if you have studied for OCA 7.

If you have time, then yes, our suggestion is that you should get a OCA 8 book and go through it. Otherwise, take OCA and OCP 7 and then later on take the upgrade test for OCP 7 to 8.

Thank you for the response!
The only thing I am afraid is that I will make mistakes due to the confusion between versions (Like interface and no body of their methods in java 7.)

Are there any points I should pay special attention to avoid it?

There are quite a few. Too many to write in a forum post. But if you follow any ocajp 8 certification book, you will be fine.