Doors can be upgraded by the door owner, paying with credits. There is one free door upgrade that is subscriber-only, ETA and another free door upgrade anybody can do. www.glitch.com/items/door/
If you are thinking of buying a credit-upgraded door, be aware that you can't buy a new door and drag it onto your already installed basic door. It would be nice if it worked that way but it doesn't. Maybe it should work that way, maybe it will someday, but it doesn't now.
Right now, it makes sense to buy a credit-upgraded door only if you are planning to add a floor to your house. Then you can buy a great door, drag it from your inventory to start your project, and bingo! Upgraded door will be in your house.
People who sell credit-upgraded furniture have run into a problem with selling upgraded doors. Doors are furniture, which means that after you make a basic door you need to drag it into your house to give it an upgrade. If your house already has 3 floors (as mine does), you can't drag a new door into your house.
There is a solution that will let anybody make and sell doors upgraded with credits. Doors, like other furniture, can be upgraded in your inventory iff they already have one upgrade. What you need is a helper, call him a Door Man, who has a house less than 3 stories. Door Man can drag a basic door from inventory onto his wall, give it the free upgrade, drag the door back into inventory, and give it to you. Now that the door has been upgraded by the Door Man, anybody who has it in inventory can upgrade it there.
I think Door Man is a new currant-earning opportunity for glitchen who have short houses, even if they have no credits of their own to play with.
ETA, my mistake, caught by ShepherdMoon! There are TWO free door upgrades, not one. No subscription is needed to do a free upgrade to Pine-Paneled Door. I edited my post to remove my mistaken idea that Door Man needed to be a subscriber.