Is CF9 compatible with CF MX apps?

Look into using the code analyzer tool on your CF code.

http://help.adobe.com/en_US/ColdFusion/9.0/Developing/WSc3ff6d0ea77859461172e0811cbec0d04c-7fee.html

We are in the process of upgrading from 7 to 9.  The only issue we found was with redbrick drivers.  If you don't know what those are you are probably unlikely to be affected.

There are changes, especially with as big as a jump from version 6 to version 9.  If sharing missery makes you feel a bit better.  I am in the final plans to change a code base from ColdFusion 4.5 (pre Java) to ColdFusion 9 this weekend!

But ColdFusion has a darn strong backwards compatibility record.  So, I would go ahead with such an upgrade with an appropiate testing period to find those niggling little code issues that may be lurking in your code.

I.E. when we tested our code base, we discovered that the developer had uses a variable name that is now a function in ColdFusion 9, "IsNull" IIRC.  Luckily it was a very easy fix to change the variable name in the code base.