I've never been interested in piranhnas myself other than if I could have a big pond full of them , there's something about large shoals of fish that appeals to me.
I'd say to soak your bogwood for two weeks before you plan to move everything, changing the water every day.
Go and pay for the pirahnas before you move anything, so you don't get disappointed.
Then a couple of days before you plan to move, wash all the sand and keep it wet and covered. Not in a tank or anything, just in a big bucket with an inch or so of water so it doesn't dry out and get dusty and so nothing can crawl in
Then on the day of the move, call ahead at your lfs to check the pirahnas are ready to move.
If they are, take all the decorations out including plants (you ownt be wanting plants with pirahnas anyway will you?) and do a thorough gravel clean. Keep one or two buckets of clean water to one side.
Refill the tank to normal level, let everything get back to temp etc and then take out the fish and put them into the buckets.
Keep the filter and heater on all the time.
Cover the buckets with lids and blankets and take them to the lfs, then spend as little time as possible with fish in bags/buckets.
Then as the pirahnas are acclimatising (I'm assuming you'll use a drip acclimatiser) put your bogwood etc in the tank so it's all ready and you won't have to stick your hands in.
Also, to trampas, I think the reason he wanted a padlock was to stop little children's fingers becoming lunch