Several weeks ago, I was asked about the compatibility between Infopath 2003, MOSS 2007 and the Forms Server. I was looking for some information about it an this is the conclusion.
In order to be crystal clear about the default compatibility between these products (InfoPath 2003, 2007 and Forms Server), you must take in consideration the following points
1. InfoPath 2003 forms will work with Infopath 2007
All the previous forms that you developed with Infopath 2003 are compatible with Infopath 2007.
2. Backward compatibility in Infopath 2007
In case you need it, you can save Infopath 2007 forms to the Infopath 2003 format. Most features should work with no problems, you can also change the default format for saving forms to Infopath 2003. In case that any component is not compatible with the Infopath 2003 format, you are going to see the compatibility issues on the design checker window (please note that if you have compatibility issues, you are not going able to save the forms to the 2003 format until you solve them).
3. Forms Server and Infopath 2003
This is probably the most common question when talking about compatibility issues between MOSS and Inforpath 2003. If you have been developing forms with Infopath 2003 and you have a lot of desktops with this version, then, you can use Forms Server that will allow your customers to open forms in the browser if they don't have Infopath 2003 installed and in case they have Infopath 200X installed, they are going to open that forms with the client.
However you should take into consideration:
Now, it is important to say that you need Infopath 2007 in order to publish the form to Forms Server, no matter if that is an Infopath 2003 compatible form. You cannot use Infopath 2003 to publish the form. The good news is that you can use one Infopath 2007 copy to publish all the forms .