Under-relaxation Factors (URFs) need to be changed only if necessary or the solution is unstable/diverging initially. The default URF values provided in the Ansys Fluent are good enough for most of the simulations. But while increasing them, the solutions may get faster but it might decrease the stability, which sometimes might lead to divergence. That’s why care must be taken while changing them. Of course, it is possible to change the URFs in the middle of the calculations. But if the URFs are set quite low, the stability will be high but it might take more number of iterations for the solution to converge. The end result will not be affected given, the proper setting of the case is done.