Delta.com says you can extend to the lower medallion tier:
https://www.delta.com/content/www/en...challenge.html
If I want to extend my Status by meeting the requirements, can I earn higher than the Medallion Tier I was matched to (for example, I was matched to Gold but I want to extend it as Platinum)?
You may only extend the same or lower Medallion Tier as the Tier you were originally matched to. For example, if you are matched to Gold Medallion Status, you will not be able to earn Platinum Medallion Status through this offer. However, if you fall short of meeting the Gold requirements during the 3 months but meet the Silver requirements you can enjoy extended Silver Status based on the dates listed.