Changing drive format for OneDrive

Which versions of OS X and Windows do you need it to work with? If they're recent enough, ExFAT is probably the best answer. Support for it was added to OS X sometime around 10.6.5 and AIUI it's built into Windows 7 and available via updates for everything since Windows XP.

BTW, if you're formatting via the Partitions tab in OS X's Disk Utility, be sure to use Options button to select the MBR partition scheme. If you use the Erase tab, just select the disk (not the volume listed under it) and it should automatically use MBR partitioning.

I'd advise just keeping things simple and clone it over if you can easily get your data down to the smaller size, otherwise do a new install and use Migration Assistant (not for any supposed performance benefits, just that it's simpler).

Fragmentation is not an issue with SSDs because the physical location of the data doesn't change the performance characteristics—each block has the same access time as any other, unlike a hard drive where it takes time to reposition the read/write head over the proper sectors.

In my experience the fresh install thing tends to be more of a Windows thing (and an older Windows thing at that). If you're experiencing specific performance issues, the consider a clean reinstall. But otherwise just clone or use Migration Assistant and enjoy the speed of a new SSD.