Are there any hints or tips to make sure an app will run fine in 2003 or 2007 ?"
Yes. Don't use 2007.
All kidding aside - although I am not kidding - 95% of code that executes correctly in 2003 will do the same in 2007. But we all know that the 5% of anything makes up 80% of any given work.
There are a few known differences that have been major problems. For example, if there is any FileSearch used in the 2003 code, this will fail in 2007. For whatever reason, Microsoft removed FileSearch from 2007 VBA, and this functionality is not available. Gone gone gone.
HarleyQuinn, here at Tek-Tips, has written a somewhat equivalent DLL to duplicate most of FileSearch. If you end up determining this was the problem, gets his DLL.
Other than that, yup, actual testing is the only solution. Obviously, this is best done BEFORE production level implmentation of 2007. Unfortunately, most organizations never bother, and end up having real-time issues when things "don't work".
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Gerry