Laying flat is a much more natural position for the leaves. They usually get bent upwards in shipping. I expect that the change in position is due to the leaves getting more floppy from minor dehydration. The main question is whether it is dehydration from underwatering or from root loss due to too much water. I would suggest watering it thoroughly in the morning and then seeing if the leaves are stiffer (more like the others, but it will probably stay flat). If it does perk up, than it was probably underwatering, otherwise, it might be a good idea to take a look at the root system. Having said all that, take it all with a grain of salt, other people may have better ideas and/or information and it probably wont hurt to wait a few days to see what other people have to say.
I hope this helps!
---------- Post added at 07:41 PM ---------- Previous post was at 07:40 PM ----------
It is also quite likely that the leaves just moved down into their more natural position just from gravity and there is nothing wrong at all.