How to transform Thanksgiving decorations into Christmas decorations?

I would just go simple on the Christmas decor. You can just add some pine cones, faux pine tree clippings, neutral ornaments, etc here and there. Then have the main focus be on the tree.

I love using white pumpkins with pinecones and little faux trees to merge the holidays a bit.

Use natural elements like willow, pine cones, acorns, cinnamon. Add golds and earthy colours or go contemporary paint Pumkins white, add gold, or copper. Don't try to mix to many different colours as it will look to much like Christmas. Soft natural fairy lights and cream candles will look nice too.

Table pieces are usually easy to make work for 2 or more seasons, with with sprigs of pine branches and acorns/pine cones etc you can put little bits of removable thanksgiving deco (like leaves, little pumpkins, turkeys etc) then for Christmas you can put snowmen Santa's fake snow etc

Wreaths are another decorations that can be easily changed from 1 season to another

Rather than getting a full size tree, you could get some downed branches (doesn't have to be big, can be any size you want) collect some pine cones, some red berries and anything else you can find that you like, and holding the branch (or pieces) of the pine tree with the pine needles facing down, attach the pine cones, berries and whatever else you collected at the center top of the stick and Hang it on the wall (I added a couple pics of Christmas stuff I made just from things I found out walking minus the lights)