Dell PoweEdge R510

Dell PoweEdge R510
We are currently using R510 with only 3 Hardrive (250GB each). We are running out of space and would like to increase the space. Whats the best way to go about it? replace the drives with a bigger ones (one by one) or use the other 5 empty spaces to increment the drive space? which option would allow us to increment space without losing any data and corrupt the Operating system?

Thanks.