Here is what I see happening
assuming Congress does not fight the President’s vision.  NASA will
start implementing this change as soon as Congress allows the FY2010
funds to be redirected.  In December, Congress slipped into their
appropriations bill that Congress had to approve the cancellation of
the Constellation program.  Congress will fund FY2011 and maybe even
FY2012 as Obama requests.  But after that they will begin to decrease
funding just like they have for every NASA program ever conducted
and/or canceled. Why do you always hear that a Government program is
behind schedule and over budget?  There is a simple answer for that. 
Because Congress only approves and releases funds on a yearly basis and
they typically cut funds from a program as the years go by.  It
happened to Apollo (change in direction), Shuttle, ISS (how many times
did Congress try to kill it), X-33/X-34, Orbital Space Plane, shall I
go on?
Words from the OMB Budget Fact Sheet
states the following: “NASA’s Constellation program – based largely on
existing technologies – was based on a vision of returning astronauts
back to the Moon by 2020. However, the program was over budget, behind
schedule, and lacking in innovation due to a failure to invest in
critical new technologies.”  To anyone outside the space agency these
sound like serious concerns and wow that program should be canceled. 
But, let’s look at the reality of the situation.  The Constellation
Program was built to former NASA Administrator’s Mike Griffin’s
desires.  Was it optimal.  No.  Was it going to work.  Yes.  Did
Constellation get funded per the original baseline schedule and funding
that was negotiated.  No.  So what happens when Congress doesn’t fund a
program?  Guess what, it gets behind schedule and over budget.  Gee,
imagine that. Constellation was based on existing technologies because
the last time this country tried to build a space craft built based on
low Technology Readiness Levels (TRLs) the problems were so great and
would take much more money to resolve that the X-33/X-34 programs were canceled.

NASA’s real problem is that its funding is done on an annual basis and
is subject to the whims of congress.  This is how our government
agencies work.  Wouldn’t it be amazing if NASA was given a long-term
vision 10, 20, 50 years and funding was baselined and guaranteed for
five years?  Imagine the programs that could be executed within budget
and perhaps even saving some money.  Imagine the R&D that could be
invested in.