Siegfried -
Using color space can be like fighting gravity - a smaller space, sRGB, cannot successfully be transformed into Adobe 98 or Pro Photo - larger spaces. One can only go downhill, I'm sorry to say. Once converted from Adobe 98 to sRGB, for example, it can't be restored without going back to the source file and starting over.
Here's a few suggestions:
If you shoot RAW, process your image files to Adobe 98 or Pro Photo
If you need 8 bit JPEGs, process them off to a separate folder.
Keep your images in 16 bit in post-production, even through printing.
Archive your master files as layered 16-bit TIFF, or if you prefer, PSD files.
Last, keep all of your good quality RAW files in archives. Software keeps improving; you may find opportunities to re-work an old image.
David Saffir
http://davidsaffir.wordpress.com