Thank you for your question to Extension. The red spots you are seeing on the leaves are probably a rust disease caused by a fungus or it could also be a leaf spot. Both diseases grow from too much water and not enough airflow (we have had plenty of rain this last month). It won't harm your rhubarb unless it spreads to the stalk. You were correct to move your plant to a sunnier location, and it's possible your plant is a little stressed from the transplanting. I would suggest removing the yellowing leaves and fertilize it each year with either compost or a light fertilizer. Here is an article explaining rhubarb care and it's diseases:

Growing rhubarb in home gardens | UMN Extension