Not enough memory to sync files to iphone

[gazbag01]

My iphone is full of music and all of my music is on my Spotify account on my pc.

i want to use the songs i have on my phone through Spotify so i can keep adding to my new found Spotify playlist but can't sync because my memory is full (with the songs on my phone)

It seems that in order to do this i will have to delete all the songs on my phone only to put them back on but through Spotify.

Is this stupid??