The long run is a period of time in which all factors of production and costs are variable. In the long run, firms are able to adjust all costs, whereas, in the short run, firms are only able to influence prices through adjustments made to production levels.
Also question is, are there fixed costs in the long run?
The main difference between long run and short run costs is that there are no fixed factors in the long run; there are both fixed and variable factors in the short run. In the long run the general price level, contractual wages, and expectations adjust fully to the state of the economy.
Can fixed costs become variable costs?
Fixed costs are not permanently fixed; they will change over time, but are fixed in relation to the quantity of production for the relevant period. By definition, there are no fixed costs in the long run, because the long run is a sufficient period of time for all short-run fixed inputs to become variable.