Some of it has already changed.  The series 1990 (and 1993)
10, 20, 50, and 100 dollar notes have had two major security
features added.  Each has a plastic security thread/ribbon running
vertically through the note with writing such as "USA TEN",
"USA TWENTY", "USA 100".  Also, around the portrait is very very
small lettering called microprinting that spells out "The United
States of America", which supposedly can't be copied by color copiers
(which is not entirely true).

  The BEP released a completely overhauled 100 dollar note in April 1996.
The portrait was enlarged and moved over to the side, a watermark was
added, and several new anti-counterfeiting features were added such as
color shifting ink (it's dark green when viewed directly head on and
black when tilted) and concentric circle printing on the reverse, so that
attempts at photocopying the note will result in odd patterns appearing
in the copy.  The general opinion among collectors is that the new $100
is ugly and has less than ideal quality printing.

  Following the 100 dollar note, a new 50 dollar note is planned
to be released in 1997, a new 20 dollar note in 1998, and so forth
all the way down to a new 1 dollar note in 2001.

  The current style notes will remain legal tender (see section 3.11).
But since the average U.S. note lasts 18 months in circulation, it
won't take long for the older notes to disappear from circulation.
Thus, it will be harder to pass counterfeit older style notes as they
will attract too much attention.

  Of course this *doesn't* mean that the current notes will become
even the least bit rare.  The old common Silver Certificates that
went out of circulation decades ago are still worthless as investments.
So don't hold your breath waiting for 1988, 1990, 1993, etc. notes to
increase in value.