Isekai should technically mean "another world". If the action is moving to the past but the world is still familiar to the protagonist, I'd venture to say that it's not suitable to have the isekai tag. This is different from other formats where the protagonist dies (in "our" world) and is reincarnated in another world s/he knows, since they literally move to a different one. But there's been quite a lot of twists and turn on the genre in the last few years, so who knows. =/