If the content of the paper is not drastically different from the conference version (same main results, but more details / data), it is perfectly acceptable to use the same title. This is the usual approach in theoretical computer science. Of course, you are free to change the title if you want to.
Examples:
Oswin Aichholzer, Greg Aloupis, Erik D. Demaine, Martin L. Demaine, Sándor P. Fekete, Michael Hoffmann, Anna Lubiw, Jack Snoeyink, and Andrew Winslow, “Covering Folded Shapes”, in Proceedings of the 25th Canadian Conference on Computational Geometry (CCCG 2013), Waterloo, Ontario, Canada, August 8–10, 2013.
Oswin Aichholzer, Greg Aloupis, Erik D. Demaine, Martin L. Demaine, Sándor P. Fekete, Michael Hoffmann, Anna Lubiw, Jack Snoeyink, and Andrew Winslow, “Covering Folded Shapes”, Journal of Computational Geometry, volume 5, number 1, 2014.
Jeff Erickson, and Amir Nayyeri. "Tracing compressed curves in triangulated surfaces." Proceedings of the 2012 Symposium on Computational Geometry. ACM, 2012.
Jeff Erickson, and Amir Nayyeri. "Tracing compressed curves in triangulated surfaces." Discrete & Computational Geometry 49.4 (2013): 823-863.