Ask an electrician, shouldn't be a major issue but with the limited info given impossible to be certain.

An electrician should be able to move them. Bring one in and see what they say.

How high are the existing receptacles?

An electrician can do it but it might not be practical to have them on the bottom of a shelf as opposed to on the back or sides since you theoretically might have books or other stuff on the shelves.

I am having a built in wall done now and am installing electrical outlets all over the unit including inside the pull out drawer that will hold my printer.

I always want to know why do you want the book cases for IMO most are used to display a bunch of stuff that people have kicking around and no thought was put in to what should be displayed. As for the electrical waht do you need it for.

Simple solution, leave the bottom shelf above the socket movable and use a junction box to extend the socket on the trim of the bookcase. This leaves the junction accessible and the socket in a useful position.

What Raye said is what I did; it works great.