When and where people are ready to notice and absorb new information is related to their expectation in the moment.
Usually, when you type a command, you expect the game to issue a direct response to that command. That response always has your max attention. So I think the most reliable place to indicate an (immediately) upcoming change to the room description in general is within that response, if that’s appropriate to the situation.
You can then also segue into the changed room description (next paragraph) by changing the start of that paragraph to how it’s always been. This works even better if the player has been through the room numerous times. They’ll be predisposed to notice a solid change to the first line of overly familiar text. Which, in turn, will cue them to recheck that whole paragraph.
I think changing the start of the paragraph can work well to indicate change even if the player didn’t just do something that directly changed the room description, but merely re-entered the room or something. It’s just the act of setting up an A>B comparison at the earliest possible opportunity.