Right now I'm using Bitcoin core and I want to use Electrum. What this procedure work with this combination?
This is a bad idea for a couple of reasons.
I will not dispute anything Danny puts forward. He is right about not being able to restore bitcoin core addresses with electrum seeds.
But I do think it is desirable to import the bitcoin core address private keys into electrum because you never know when you will receive coins into the old addresses. I would keep the keys of those old addresses in electrum and not use them. Only delete them in the future when I am 100% sure.