You start from last time, not from a blank page. There are two routes, and neither uses credits; the first request in the new document does.
In the library, choose “Use as template” on last week's deck. It copies pages, page format and images into a new document, not chat, versions or attachments. On your first request the AI keeps look and structure and rewrites the content page by page, so you attach the new status export and say what changed.
Or open last week's PowerPoint file with “Open PowerPoint, PDF or Word”, up to 30 MB. Importing keeps speaker notes and slide size. On paid plans, slides you leave untouched are normally taken straight from your original file when you export, as long as the original stays attached. New and changed slides are ordinary text boxes, shapes and pictures in your design. The page on improving a PowerPoint with AI has the details.
Compare every figure with your source before the deck goes out. The AI is told not to invent figures and to label its assumptions, and the closing chat message names the material assumptions; read it before you read the slides. The first result is a complete first version to refine.