I have a big website....
If the current titles and descriptions are "close" but not "optimal" then small tweakings will probably not be a problem.
But... my concern is that you have a large website and plan to do this job in one day.
I often spend a bit of time on each title and description. I want a title that presents the the page for the proper keywords (so I do a bit of KW research). I also want a title that elicits clicks or presents a value proposition (so I spend more time on that). Then I want a description that personalizes the article to the search audience, or presents secondary keywords and secondary propositions (again, more time).
These things can take some time, but, if done deliberately and well, they can multiply your traffic potential beyond what you might get from a "slap them on quickly effort".
So, I would spend 1/2 day per week over the next couple of months on this job. I would do the pages with the highest anticipated value first, then work my way to the less important pages, while keeping an eye on the performance of those most important pages.