The Collaborative International Dictionary of English v.0.48:

metrication \met`ri*ca"tion\ (m[e^]t`r[i^]*k[=a]"sh[u^]n), n.
   The act or process of converting the standard system of units
   of a country to those of the metric system; metrification;
   as, Great Britain has completed its metrification, but the
   United States still resists complete change.
   [PJC]

WordNet (r) 3.0 (2006):

metrication
    n 1: the act of changing from imperial units of measurement to
         metric units: meters, grams, seconds [syn: metrification,
         metrication]