Have you a screenshot to show off the effect you meant ?
clear line between the 2
Hi-res textures can consume huge amounts of memory, especially on memory limited devices like consoles. In this case only low res-textures are often loaded for potentially visible geometry. When you come close to the geometry the hi-res textures are loaded in the background (streaming) and slowly blended in to exchange the low res version. This is a continiously process in which no longer used hi-res textures are overwritten by new hi-res textures.
What do you mean by "only makes sense when streaming in hi-detail textures"?
A simple way to accomplish this is to use two samplers during the blending (exchange low res<->hi res). Once you have faded it in you can either use an other shader, a dummy texture, branching etc. to turn it off.