You can move from L1 to H1B anytime of the year, citing your previously approved H1B petition.
Both L1 and H1 tenures are taken for calculating maximum stay period of 6 years. Hence you could stay another 2 years and 3 months in new H1B from Dec2012 - Max 6 years minus (1 year and 9 months in old L1 (Aug2008 to Apr2010) plus 2 years with the new L1 (Jan2011 to Dec2012)).
Hope that helps.