Depending on the features of the patients who are transplanted (ie age and underlying disease), the incidence is between 0.5-1.0%. Here is the link to one large published series (there are other reports in the literature)-
If there is no sign of metastatic disease I am not sure you have to make significant adjustments to the immunosuppression at this stage. Management of metastatic disease should be multidisciplinary with input from an Oncologist.
Article Renal Cell Carcinoma in the Native and Allograft Kidneys of ...
The only experience I have is ; male in 50s who had abnormal behavior and suicidal attempt with prazocin overdose at post KT 5 th month ; found to have multiple haemorrhagic brain metastasis on non contrast brain CT ; after he had a focal seizures with secondary generalization leading to status epilepticus . Possibility of brain abscesses and cerebral toxoplasmosis had been ruled out . Patient died after a week and relatives did not give consent for a pathological postmortem .we couldn't confirm the primary site : but possibility of RCCA was there .