Can band be downgraded from last year B to current year D? having 100% wfo. MLT cleared. but got unit change

Did u do something horrible to cause huge negative impact on TCS or client? Did you violate any policy?

If answer to any of these is yes, then sure, you are getting a D. But other than that, most probably it is a B or if you pissed someone off or committed some negative impact of mild nature like some escalation or something then it is a C

anything is possible here

Band should be based on performance. So yes, you can be given a D even if you got A last year.