2003 Chrysler Town and Country

Engine Mechanical problem
2003 Chrysler Town and Country Automatic

Can you swap a 2003 3.3 engine for a 2006 3.3 on a Town & country Van? Someone told me it could work but when it was done, though it fitted well, it would not run. What could be wrong?

Or is it just that a 2006 cannot work in a 2003 van, Period? Help, before I make another mistake as the mechanic wants me to take out the 2006 that's already installed and not working and have another engine, this time a 2003 put in, with all the additional expenses of course.

Thanks.