Years ago, I used to hang "regular" wallpaper professionally, so I can
only respond to that part of your questions, but I would imagine the
reasoning to be the same for "paintable" wallpaper.

First, wallpaper can be painted over. You would want to prime it
first, especially if the paper had a definitive or bold pattern or
color. This is generally the preferred route for most people wanting
a change. The second option to paper over old wallpaper (see below).
In either case, check to make sure you don't have more than 3 layers
of pre-existing paper.

Second, wallpaper can be wallpapered over again. In many of the older
homes that I have hung paper in, there were layers upon layers upon
layers of wallpaper, which I would systemically refuse to remove
and/or paper over. Ideally, you do not want to paper over more than 3
layers of existing wallpaper; it becomes too risky and too difficult
and patchy to remove. The combined weight of the paper and glue makes
it extremely heavy, and three layers are about all you really want to
push before damaging the wall.

If you do intend to paper over old wallpaper, I'd suggest adding a
little bleach to your glue--that helps keep the mold in check.