I have recently taken the plunge and changed up from 9.7 to 9.9.
Most of my apps compiled without a problem (all OK in 9.7). I had a number of forms that were reporting errors and would not compile - the errors were long and complex and referenced adodb - the database extraction layer.
I found several patterns:
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Checkboxes not defined. I had a number of checkbox datatypes (from boolean data) - they were not displayed in the form. Once I changed them from Automatic to manual and defined a value they were fine. It woruld probably also work just changing the datatype to INT. I then moved them back to being not shown again.
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Nested SELECT statements in a look up. Ihad a few SELECT fields where the SQL nested one statement in another (e.g. SELECT staff_id FORM staff_list WHERE staff_id NOT IN (SELECT staff_id from another_list)) - It really doesn’t like this. Re structuring teh query so it was a join and not nested fixed this.
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Misbehaving many-to-many relationship that wasn’t as fully formed as 9.9 wants with some fields not correctly set up.
In general a strategy that worked, shown to be by Joao at SC tech support, was to revert each field just to being TEXT and then recompiling - this usually works and then one by one adding back the fields to their correct datatype (eg. CHECKBOX or SELECT) to find the problem one.