The good news is that you've lost a lot of weight. The diet has worked and you're now in shape. The bad news is that your current work wardrobe doesn't look quite right. And unless you want to look odd, you will need to do something about it. Your shoes and socks are going to be the only items you don't need to deal with. Everything else needs to be addressed. Here are a few ideas to help get you started.
A big baggy shirt is a bad look at the office. You don't want to look unprofessional, which is what will happen if you wear a shirt that is billowing around you. So, unless you have bespoken shirts from a Savile row tailor, it's best to just donate the shirts to the less fortunate and then get yourself a new set of shirts. The process of re-cutting shirts is, in most cases, not a great idea, unless you originally purchased custom shirts. You might as well just get new shirts that fit. While you are at it, you might decide that you like a slim fit shirt. In the past you might have worn classic fits, so now is the time to consider new fits.
Get Your Belt Re-Cut
If you wear leather belts, then you should get them re-cut. Don't make the mistake of trying to drill new holes into the belt. It will look odd. You don't want the belt to pass through multiple belt loops left of the buckle, at least not for office and dress wear purposes. The cinched look will make it appear as if you're wearing clothes that don't fit. So, you should have your belt re-cut. This needs to be done at a cobbler (a person who works on shoes). They can cut the belt at the top (near the buckle) and shorten the length then resew the leather so that the belt is a proper size. And if you do have an expensive leather belt, it's going to be cheaper than just going out and buying a new leather belt.
Suit Alterations: Slacks
The most important wardrobe alteration is going to be your slacks waist. You will want to go to a tailor and have them taken in. You don't want to wear big baggy dress slacks, and you also don't want to cinch the belt tight with a belt. The solution is to visit the tailor and have them fix it. You bring the pants and try them on in a dressing room, then the tailor will mark up the pants and you will hand them over. The pants will then be taken in and fit properly.
For more help, talk with someone like Fifi's Tailor & Custom Design.