What are the situations which can cause a resistor to permanently increase its resistance?
Carbon resistors (e.g. anode loads in valve amplifiers) that run hot, dissipating at least half of their nominal power rating, can increase in value by 30-50% or so over prolonged use, and should be replaced every 40 years or so.
In that case, prolonged operation at high temperature is the problem. As you say nothing about the type of resistor, or the power rating, or the voltage, this may or may not apply to your situation.