Downgrade from 3.4 build19274 to 3.3 build19052

I am now in 3.4 build19274 and I want to downgrade to 3.3 build19052. With Ford Sync Update Downloader, I prepare two USB, one with downgrade files and the other with 3.3 build19052 files. My question is: firstly I put USB (downgrade files) into the car till the end of process and then put USB (3.3 build19052 files),correct? Will the whole process take long time?