Is there any way I can re-format my external hard drive to accept larger data files without erasing the current stored data on the drive??
A reformat, by nature, destroys all data on a disk. There's no way of avoiding this.
If you need to change the filesystem on it (e.g. from FAT to NTFS or some other change - I assume something along these lines is the nature of your question?), then by far your best bet would be to back up everything onto another device and then copy back after the reformat.
No, if you format a Hard-drive you should backup your Data.
If you want to move from ext3 to 4 that is possible without having to format the disc:
But even then you should always back up your data somewhere else in case the process goes wrong and your data is lost.