It depends on WHY you would want to change the transmission. If it works fine, maintain it scrupulously and drive on. If it's showing signs of wear, then failure is probably not far off. It's worth having it checked out even if there are symptoms of wear, sometimes a good flush is all that's needed or it could be a simple electrical part or other external part. Transmissions are EXPENSIVE; no need to buy one unless you REALLY need to.
Take it to a mechanic
Yrs
That means time to change transmission
The 2008 Honda Odyssey has a 5-speed automatic.
The 2013 Honda Odyssey has a 5-speed automatic.
The 2009 Honda Odyssey has a 5-speed automatic.
The 2007 Honda Odyssey has a 5-speed automatic.
The 2001 Honda Odyssey has a 4-speed automatic.
The 2012 Honda Odyssey has a 5-speed automatic.
The 2005 Honda Odyssey has a 5-speed automatic.
The 2003 Honda Odyssey has a 5-speed automatic.
The 2014 Honda Odyssey has a 6-speed automatic.