The equations for energy balance and heat transfer can be used to predict changes in performance. For this level of estimate it is probably safe to assume a constant value of overall heat transfer coefficient.  This assumes no significant change in flow rate of either fluid -- corrections can be made if you know the change in flowrate of either fluid.

Sensible heat transfer energy balance: Q = Mh*Cph*(Ti-To) (hot side) = mc* Cpc *(to-ti)  (cold side)

Heat Transfer:   Q = Uo * Area *  LMTD