Import products are pastuerized and therefore can be stored at warm ambient temperaures. Most of the brewers flavor qualify their products to determine what the shelf life of the product after packaging would be before they flavor deteriorated. This seems to be consistently around 120 days or so.
The key to taking advantage of the shelf life is to set up the dispensing system correctly so that you are not allowing the carbonation to leave the product nor are you imparting CO2 into the product. You need to gain equilibrium with the gas in the beer with the pressure you are applying to it.Know the temperature of the beer and the carbonation level of gas content in volumes or grams per liter and apply pressure accordingly. Example: a 2.5 volume (5 G/L)beer @ 38 degrees F requires 14 PSIG at the keg.
All domestic draught products are non-pastuerized and must be kept in cold storage at all times or the flavor will be damaged quite rapidly.
As to switching kegs, they are a sealed vessel once uncoupled and will keep quite well as long as you abide by the equilibrium rule above.
Scott Zuhse, Instructor
Micro Matic Dispense Institute