Rebuilding to a larger disk will always result in the same filesystem as the original since parity is at the bit level not the file level.
To change filesystems you have to reformat, and so will have to copy the data somewhere else before reformatting a disk. Since you are going to larger disks you can just rebuild one disk, which results in a larger ReiserFS disk with all the same data but with a lot more free space, then copy or move the data from other disks to that free space. After emptying a disk, you can reformat it to XFS and then rebuild it to a larger disk, or rebuild it to a larger disk then reformat it to XFS.
Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.