Depends what the situation was. Piper rode in a backless booster (rated from 40 pounds) at just over three years old when our neighbor took a header into our coffeetable and needed a staple. She was bleeding pretty badly, we couldn't see exactly what was going on, so we got into the mom's minivan. The neighbor went in her backless booster (at only 34 pounds herself, I got them to buy her a high back booster), the cousin went in the third row where she luckily passed the five step test (at age six), and Piper went into the cousin's backless. It was a mile and a half up the road and back. Piper did just fine, holding our neighbor's hand (who was NOT doing fine, she was leaning forward and moving about, and she's so little a locked seatbelt doesn't do much to keep her where she needs to be. Even now at nearly 7 years old). So for blood and a head injury, yes I would have let Piper ride forward facing for the ride there at least (can rear face when things have calmed).
If it's a question of needing a diagnosis or treatment? Then probably not. I'd reinstall the seat. Something that can wait two minutes.
Wendy