You need an area safe to keep them for a couple of days - sand is notoriously messy to swap over to.
I did it just a couple of weeks ago, and this is what I did:
ALways have a spare tank - a cheepo plastic thing will do.
Syphon tank water into the spare, about 2/3 full.
Pop the bigger bits in here, like plants and stones in, then carefully but firmly grasp your 'Lotls, and quickly transfer them into it.
You'll probably need a small bubbler to keep the water moving slowly, and oxygenated. Pop the filter in here too - make sure the biocycle won't die see.
Syphon the rest of the water into your waterchange bucket - keep one bucketful.
Remove the other substrate, pop everything on a waterproof surface obv!
Wash sand, and layer slowly in the bottom - about an inch is fine.
I used about 1.5cm - but Spyyk likes to kick it about anyway.
SLOWLY pour in the bucket of tank water - it WILL go cloudy.
Top it up a bit with fresh dechlor - make sure you'll be able to put a quarter of the water in the 'lotl backup tank back in later.
When it's the same temperature as the backup - put the filter back in to aerate.
Put a fine filter pad in to help clear the "dust" out of the water - clean once every two hours.
Pop the plants back in, and big ornaments.
Leave pebbles and stuff for the axies to play with, and do a waterchange in the morning - nasty chemicals WILL build up.
Watch out for the pH rising in the main tank - sand is slightly acidic.
After it's cleared, they can go in - with a fine filter(every 2 hours), it'll take about 36 hours.