Vic
An earlier reply gave a pretty complete procedure for changing tanks and although your question related to changing a tank; may I add a similiar procedure for the end of the day - if the truck is not going to be used overnight or for the weekend.
Pretty much the same:
With the engine at idle, turn the tank off, rev the engine til it stalls, try to restart the truck 2 or 3 times then shut the key off.
In the morning, give the tank 1/4 to 1/2 turn, start your truck and with the truck idling, slowly turn the valve on all the way.
The key words here are slowly and all the way.
PS: If anyone stores their propane lift truck in an unheated warehouse and have a problem with starting - especially on a Monday morning.....try this shutdown procedure and you should be pleasantly surprised.
Regards
Garry