Is it safe to resize a vfat partition from the start sector?

Resizing a partition from the start sector can take much longer that from the end sector. This means that it remains exposed for more time.

Every manipulation of the partitions is more or less risky, at least because of hardware or power issues. During a longer operation the hardware is much more stressed, so the operation can be considered as "more risky". That's why we always advice to keep backup of any important material (data files can be more important than operating system, I think). Most of the times there is no problem (there are few issues reported to the support forum, mostly concerning boot issues and very few about data loss). It is better to spend some time to take a backup.