A Technical Upgrade is also called a C/SIDE, executables, engine or client only upgrade where you're just upgrading the client executables.
Why would you do a Technical Upgrade?
Doing a full Navision upgrade means doing both an Application Upgrade and a Technical Upgrade. This is often a long (sometimes very long) project, especially if you have a lot of customizations and many users who might need new training.
Doing a Technical Upgrade will not give you the inprovement of the application (such as new fields, reports or functions), but it does give you improvements of the new client, such as C/SIDE features and very often performance optimizations.
Is it always safe to do a Technical Upgrade?
Except for a few versions then it has always been possible to perform a technical upgrade without having to upgrade the objects. But many version have smaller functional changes of the C/SIDE features. Often these changes are actual fixes of errors or removal of "undocumented features" that the developers have been used to and there have been using in the code. This the case both for standard code, but especially for customized code.
Therefor if you are only doing a technical upgrade then you must still do a full end-to-end test to make everything still works as supposed.
Upgrading to Microsoft Dynamics NAV 2009
Because of the pages and the new reports in the new role tailored client, then if you are doing a technical upgrade to Dynamics NAV 2009, then you can only use the classic client.