Switching your pedals this close to a ride that you may already find to be difficult is probably a bad idea... stay with what you are used to as you won;t be able to dial in your fit on the fly.
If you have been riding 30 miles at a time and doing this on a regular basis and the 50 is not unduly difficult you might do well over 100 miles.
You can live with a less than perfect fit over 30 miles, but over 100 miles you will find out what is right and what needs to be changed.
The only way you can find out if you can do it is to do it and expect that you will have support along the way and many be able to ride with a group and save a little energy by drafting... just as long as you can do this safely.