How do I to convert YUV420SP to YUV420P using ffmpeg sws_scale or another efficient method?
There are an enormous amount of different YUV-formats available, as a starting point see http://www.fourcc.org/yuv.php
You need to be more specific in you question since "Semi Planar" and "Planar" doesn't really tell how the data is formatted.
But as a general advice, it's just raw-data placed contiguously. All you need to do is to read the correct amount of data from the in-stream and write it to the out-stream using your favorite language.