Idea

"Assuming that the as a modeler I did not assume anything about the units"

That's a big assumption, and most likely false.

For example, any objects in the model have sizes, and if you change the length units, then now they are the same size but a different unit. For example, your processor may now be 4 inches long instead of 4 meters if you change the length units from meters to inches.

Also, any task executers in your model have speeds, accelerations, and decelerations. If you change your time or length units, then those values are now unchanged but mean something different.

Also, any process times or other values in code that aren't explicitly wrapped in unit functions will be the same values, but the units will be different. For example, senddelayedmessage() calls, source arrival times, time tables, mtbf/mttr downtimes, etc will be the same values but different units.

Manually changing the model units in the tree is fine, as long as you recognize that changing those values doesn't affect the values of any existing objects in your model, but it does change what unit those values represent.

Also, the default values of objects in the library (such as sizes, speeds, etc.) are based on your model units when you start a new model or open a saved model. So if you manually change your model units, you will need to save and reopen your model in order for the default values in the library objects to update to your new units.