No matter what, you should ALWAYS back up your data before trying to expand any volume in any way. We had a customer who tried this (in his RAID controller) and accidentally selected the wrong option and it wiped his data on that system. He was able to recover his data from another open-e system that had some active snapshots, but it was still a hassle (and quite scary for him).

ALWAYS backup before you do this sort of thing! Is it worth the 10% chance that your data will be lost?

Do I loose data on the old harddisk (which would cost my job)
Frankly, if you don't backup your data before trying this sort of thing (even if we tell you your data will be fine), you should probably lose your job, even if the data is fine!

ALWAYS have a backup, unless you don't care if you lose data. And RAID alone is NOT A BACKUP.