Whether you can use lead-free paint to paint over lead-based paint depends on if the house is fully encapsulated. Once the paint starts to deteriorate, the risk of lead dust spreading is probable and you should no longer paint over the lead paint. If the house is starting to show signs of paint failure, such as peeling and blistering, then the paint may be releasing lead dust influencing you to get the lead removed. Since we are dealing with a toxic chemical we must follow all EPA, RRP, and OSHA rules.