I really like both names.
I approve of changing it to
Cassandra. I think it's cool that you can make your name feel more complete and still keep the name your parents gave you. The only issue is, and it's just barely an issue - I pronounce
Cassandra ka-SAWN-dra, and
Sandra SAN-dra. I guess if you pronounce the middle A the same in both, it's not an issue at all.
If you're wanting to get rid of your mother's surname for some reason, it sure is a convenient thing, to expand your first name at the same time. I actually think you'd need a better reason to drop your mother's surname (whatever your reason is), than you do to change
Sandra to
Cassandra.
- mirfak