Hi Irina,
Your absolute best approach would be to create a test environment. Clone your database and MOSS environment into a separate area then run the upgrade to make sure nothing breaks.
I would never recommend doing it any other way; too many unknowns and your risk for something going wrong in the production environment is too high.
If you do find everything works well in your test environment, then move to the production environment. However, I would make certain you make a full backup test the recovery process so you can roll back in case of problems.