In order to maintain the level of carbonation that you achieved, the headspace CO2 pressure (rigorously the CO2 partial pressure) has to be maintained at the pressure that is in equilibrium with the current carbonation level at the current beer temperature - i.e. the "chart" pressure. If the headspace pressure is reduced from chart pressure, then the beer will lose carbonation. If the headspace pressure is greater than chart pressure, then the carbonation level will increase.
If a beer is fully carbonated, and in a leak free keg, then CO2 gas can be disconnected, and the headspace CO2 pressure will stay steady (unless the temperature changes.) No additional CO2 will be absorbed by the beer from the headspace, nor will CO2 escape the beer into the headspace. If for example, the keg temperature is increased (while disconnected from the CO2 tank) then CO2 will diffuse out of the beer into the headspace, thus increasing the headspace pressure, and lowering the carbonation level. If you then cool the keg back down to the original temperature, some CO2 will diffuse from the headspace into the beer, reducing the headspace pressure, and increasing the carbonation level. The process is completely reversible, since the total amount of CO2 in the keg is constant.
Once you start serving, you have to replace CO2 in the headspace in order to maintain the equilibrium CO2 pressure and carbonation level. If you don't backfill CO2 when serving, your carbonation will slowly decrease, as more and more CO2 diffuses from the beer back into the headspace.
Brew on