Is it possible to transition from polyurethane to silicone?

when replacing existing polyurethane with silicone, great care needs to be made and a lot of extra labor is involved to ensure all existing material is removed.  If the polyurethane is left trapped under the new silicone, it can deteriorate the new silicone over time which can destroy the bond and material. 

At PCM Services, we always recommend pre-construction mock-ups and testing to determine the level of preparation work required to do the job, to determine if a primer is necessary, and to ensure the aesthetics are acceptable. Many studies have shown that the average typical construction job budgets less than 1% for exterior sealants. And yet, after construction, close to 80% of all water intrusion issues related to the original construction are due to joint sealant problems. 

There is both a science and an art to successful completion of a functional sealant joint. In the end, it will cost much more to replace the sealant than if it was simply installed correctly the first time.