I jsut did the switch in my 50g. I did it all at once. Took my fish out, and put him in a bucket with tank water, PH, and heater for a day. I scopped the CC out, threw it away, and added LS.
I have 27x turnover in this tank, and it took overnight to get crystal clear again. Make sure that you are using carbon filtration, and not a micron cartidge...that was my biggest mistake. Just so you know, I did experience a spike in ammonia, causing a mini cycle. Everything turned out OK, and I only lost one Mexica Turbo Snail.