F1 Stamped for 5 years. Is my f1 still valid ?

I came to US for Masters degree in Dec 2010. I worked on OPT till Jul 2013, left the job and returned to India then. My employer applied for H1(in April 2013) and it got approved. My sevis is completed as my h1 is approved. No H1 stamping yet. I have never worked on H1 as I left before October.

I have F1 visa stamping for 5 years, expiring in Nov 2015. I am planning to do PhD in US this year from a diff University. Do I need to go for F1 stamping again ? Please suggest. 


Thank you.