As soon as you get in a new boat on an unfamiliar run, go upside down. Roll up and you'll feel much better knowing that at least you can do that. Then work a few easier rapids and walk the ones that you're not sure about or that don't have a good pool at the bottom to pick up your stuff.
Having a couple days in a new boat in easier whitewater should be enough to get comfortable with it up to your skill level as if it were your own and know whether pushing it to the next level will be a good idea or not.
My first time in a Jackson (hero) was putting in on the Middle Fork which was at the upper end of my skill level and it turned out to be a weird boat compared to my Mamba, but things worked out and was very comfortable in it within a few days. The boat felt unfamiliar and strange at first, but hitting my first roll in the put-in eddy felt great and hitting my first combat roll in the first rapid after I flipped because I wasn't familiar with the boat's edges and secondary stability was an even more triumphant feeling.