If your developer modified any of the DotNetNuke framework's core code or core database tables/stored proceedures, there will indeed be a high probability that such work will be lost as it will be either overwritten during an upgrade or cause the upgrade to fail leaving the site unusable. That is one of the primary reasons that we recommend that no core code or database modifications be made.

If your developer modified code and database objects of only "core" modules (those included in the framework package but not automatically installed) or other third party modules, those changes should not be overwritten during the upgrade. However, those changes might have introduced incompatibility issues with later versions of DotNetNuke, particularly if direct references were made in SQL scripts to core database objects which are not guaranteed to remain unchanged from framework version to version.

At any rate, I would first recommend cloning the live production site (files and database) to a localhost installation then attempt the upgrade. You'll quickly find out what breaks and what doesn't and can then determine what modifications will have to be reapplied or changed when doing the actual upgrade. Also, before doing the actual upgrade on the production site be absolutely certain you have good backups of BOTH the site files and database.

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner