I imagine there is not enough precedent that a specific style has emerged, but you never know.

I've written it as:

INT. LECTURE HALL / ATTIC - DAY

In the scene the character is hallucinating the lecture hall and it gives way to the reality of the attic.

Should I do it differently?

I notice you didn't get a response to this. Sorry for the delay.

I would approach this differently than the way you've presented it with dual locations in the one slugline.

Start with where we are, after all this is a guide to where you're crew are shooting, and it is also what your audience will see. So, we are in a -

INT. LECTURE HALL - DAY

Focus on some detail your character is looking at, or hearing then have it

MORPH into -

ATTIC - NIGHT

If your character is hallucinating we will at first see what he is seeing and the location he sees. This is your chance to create a pretty nifty visual - and after all that's what film is all about.

Have a look at Nolan's Inception screenplay, specifically the scene where we are first introduced to Ariadne with Cobb, where dreams mix with reality but are definitely written as separate locations.