Promoters and enhancers-key controllers of gene expression-have long been distinguished from each other based on their function. However, recent work suggested that common architectural and functional features might have facilitated the conversion of one type of element into the other during evolution. Here, based on cross-mammalian analyses of epigenome and transcriptome data, we provide support for this hypothesis by detecting 445 regulatory elements with signatures of activity turnover (termed P/E elements). Most events represent transformations of putative ancestral enhancers into promoters, leading to the emergence of species-specific transcribed loci or 5' exons. Distinct GC sequence compositions and stabilizing 5' splicing (U1) regulatory motif patterns may have predisposed P/E elements to regulatory repurposing, and changes in the U1 and polyadenylation signal densities and distributions likely drove the evolutionary activity switches. Our work suggests that regulatory repurposing facilitated regulatory innovation and the origination of new genes and exons during evolution.