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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 is planning on releasing a completely overhauled 100 dollar note in 1996. This will probably have the portrait moved over to the side, an added watermark, and several new anti-counterfeiting features. As of March 1995, this plan is still on schedule.
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 presumably 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.
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