I think the "company" Maria Lucas is used to are mostly people she has known all her life, while Lady Catherine is a total stranger to her, and financially a step above most of the people back home.

] Perhaps Sir Lewis just replaced the windows?

I think that's it. Old houses certainly need repairs and parts replaced. If you read the histories of some of those English "stately homes", they're full of examples of their various owners through the centuries renovating, making additions, etc. as their needs or the fashions changed.