Got it!

http://www.daz3d.com/forums/viewthread/22550/

It's the answer by DAVIDGB, posted: 25 May 2013

For convenience I'll quote him here (hoping that's okay with him):

Quote - Many people don’t realise this, but DS already contains a tool to convert textures between different UV sets that the same figure has.

For example, let’s say you have an M4 texture set you want converting to the M5 UVs, In DS, if you have Genesis with the M4 and M5 UVs (of course you get the Genesis M4 UVs with the basic content, but get the M5 UVs by buying M5) you simply use the ‘Map transfer’ command in the surfaces menu, and DS will convert the M4 textures to the M5 mapping (that is, DS generates new jpegs of the textures, with the textures transformed, like with Textyre Converter 2 etc).

DS has actually had this function since DS3 and pre Genesis. If, in DS3, you were to load V4, say, then select the M4 obj as an alternate UV set for the V4, DS would convert V4 textures to M4 UV mapping and vice versa. The simple requirement for DS to convert textures from one UV mapping to another is that one figure (or prop) must have both the UVs to convert from and to available for it. With Genesis, of course, that means textures can be converted by DS between the V4, M4, V5, M5, A5, S5, H5, F5 UVs, plus those for the various Genesis custom creatures that come with separate custom UV mapping. But Genesis is not itself a requirement, just that a single figure or prop has both UV sets available for it.