After you shop the local Propane providers for the best prices and the best “new customer” deals, make two phone calls.
(1) Call the old supplier and tell them to come get their tank.  They have no motivation whatsoever to be cooperative and speedy, so they will show up when they want to, probably after you nag them with several follow-up phone calls.
(2) Call the lucky new provider and tell them to bring you a new tank and some gas.  They will arrive quickly, remove the competitor’s old tank, and, happily, set it somewhere.

They will install your new tank.

Then a tanker will arrive and fill your tank.  Most residential tanks in California have a 250 gallon capacity.  They will fill it the first time at about 85% or 200 gallons.
The technician(s) will open the valves and hang around for a few minutes while the propane fills the empty pipes in your system.  This is called “charging” your system of pipes.

Then they will test your system for leaks with a water manometer that they will attach at the regulator.

If the manometer holds steady everybody shouts “Hoorah!”  You are leak-less.  But if the liquid column in the manometer drops, then you have a leak.  Somewhere.
Where is the leak?  Finding that leak can be quite a project if you have a lot of propane appliances in your home:

• furnace
• hot water heater
• gas fireplace
• cooktop
• oven
• barbecue
• clothes dryer

If you do have a leak, as indicated by falling pressure in the manometer, the technician(s) will get out an old fashioned squirt bottle or an electronic sniffer or both and start tracking it down.  Sometimes, after they locate the leak, if it is a simple tighening of a loose fitting, they will fix it for you, but they may stop, turn off the system entirely and tell you to call the plumber or heating-and-air guy or other tradesman.  The propane guys will return only after repairs are made.
So, lets hope they don’t find any leaks in the first place.
Or, if you do have a leak, let’s hope they DO find it!
By the way, you should be asking this question:  just how, exactly, do they remove the old 250 gallon tank and replace it with a new 250 gallon tank?  These tanks weigh hundreds of pounds.
If the tank is sitting on a pad conveniently near a street or driveway, or the ground is firm and relatively flat, they drive the truck up next to the pad and lift the old tank off with a hoist, put it aside, then lower the new tank into place with the hoist.

But, the pad is almost never next to a driveway or street.  The tank is usually up a hill behind the big tree where it can’t even be seen from the street.  What then?
They drag it.
Yep, uphill.  Across the lawn, through the weeds, 3 or 4 guys drag the new tank up to the pad.
How do they move the old tank down hill?  They slide it.  Wheeeeeeee!
Now you know a lot more about propane tank switching than you did a minute ago.
Aren’t you happy?