Tent swapping is a late game strategy to improve your economy, and/or oil production. There are two different types of "swapping" - either selling technology buildings and replacing with HQ's (to improve your economy), or selling Infantry and/or Airborne HQ's, and replacing them with Armor HQ's (to improve oil production). Lets have a look at both of these techniques:
 

Some technology buildings are no longer necessary in the later stages of the game. Which buildings become expendable will vary depending on the type of regiments you wish to produce, or in some cases by differ by country. A perfect example that applies to all countries is the engineer brigade. After you have researched all desired engineer brigade upgrades, and built all necessary base defenses, you can sell the engineer brigade and replace it with a HQ to improve your economy. All upgrades you have researched from there will still be in effect, you just won't be able to produce anything that requires the engineer brigade, such as AA vehicles (don't make those anyway), engineers, airfields, etc... Other applications include:
 
Heavy Tanks: If your late game strategy is just to produce heavy tanks, do your artillery brigade upgrades and then sell the brigade and replace it with an HQ.
 
Arty Tanks: If your late game strategy is just to produce arty tanks, do your motor pool upgrades and then sell the motor pool and replace it with an HQ.
^{NOTE: Japan requires both the Motor Pool and Artillery Brigade to produce Arties}
 
Airfields: After you have built any desired Airfield(s) you no longer need your engineer brigade. Airstrikes are expensive, so replace that engineer brigade with a HQ asap.
 

This is a technique that is often misused, and should only be done by Japan and the United States, and even when done by them it's result is somewhat questionable. The Armor HQ's of Germany, Great Britian, and Russia all generate LESS money than their Infantry and Airborne HQ's, so tent swapping with them would harm your economy.
 
So if you are Japan or USA, this is how the idea works. When you get your economy started you spam alot of Infantry HQ's because they are cheap, and the fastest way to get your economy going. Not uncommon to see bases with 10+ Infantry HQ's, and it's certainly not because they plan on making that much infantry!
 
Armor HQ's generate +10 more oil than Infantry HQ's, so the idea is to sell Infantry HQ's and replace them with Armor to generate more oil. While this theory sounds good on the surface, it's a very expensive way to increase oil production, and as mentioned before will even hurt your economy unless you are Japan or USA. Heres an of example:
 
USA: Sell 5 Infantry HQ's, and you are refunded half of their original cost: +$625
Purchase 5 Armor Tents -$1750
Result: +50 Oil - Total Cost: $1125