If I use the option under my account to swap out one of the theme packs that I am already subscribed to for another how long will it take until the new channels are available? Is it immediate or will it not take effect until the next billing cycle?
It should take place in a short time after you make the changes. Your next bill should be prorated for the changes you make if there are pricing differences.
I changed mine just the other day and by the time I'd turned my Optik box on and got to the guide they were already there.
So a matter of minutes.
As has been mentioned, the charges are pro-rated meaning you will only pay for the portion of the month that you had the package.