I suggest that you begin by looking for coordinating fabric that would work with the rest of the room. That might dictate which of the options would look best. I have no experience with lined drapes so I can't help with the actual technique. Good luck, hope you find a way to make it work.

I don't sew, so I don't know how difficult re-doing the bottom hem would be. If it's at all do-able for you, I'd pick option #2.

I make all my own window treatments, and would advise going the add fabric to the bottom route, which isn't hard.

You'll need to do it to the lining, too. It MAY be tough to get a fabric that you won't see through at the seam that joins the bottom, but it looks like you have the perfect set-up, with the length of your current ones, to emulate your #2 'add to the bottom' pic above, as they change fabrics at the point the window ends.

Maybe someone here with Photoshop skills can do a mock for you if you post a fabric that you like. I think it would be easier for them if you opened them a bit more, so more of the current fabric is shown, and took a head-on pic of the window, down to the floor as your first pic is. Good luck!

Thanks everyone! Doing the bottom hem isn't too hard, just more time consuming than not doing one :) But if people think that would be best, I'd attempt it. Regarding comments on choosing coordinating fabric, I think I would choose a teal silk of the same type - I went to a couple fabric stores over the weekend and don't think it will be too difficult. I have teal accent chairs in the room and I think this would make the look more cohesive. Either teal or a gray, but I'm kind of tired of gray and want a cozier look in the room.

If I add to the bottom, the next question is: at what point do I start the new color? I might be able to line it up with the bottom of the window sill since I will gain about 3 inches in length once I undo the bottom hem. But if I don't have enough, I think it would look bad to have the transition be a few inches above or below the sill (e.g., look like I measured wrong). In that case, how far up the window should the transition be?

AMS, the teal sounds nice. I had a feeling that you wouldn't have broached the subject if you didn't have a particular fabric in mind : ) .

Not only do I not sew, but I'm not a designer, so I'm not entirely sure about where to start. I would do a Google Image and/or Pinterest search for curtains in rooms by designers (real designers, not design bloggers) to see what's been done. When I did that, I came up with this example from Sims Hilditch; more views here,

https://www.simshilditch.com/interiors/lateral-apartment/

Honestly, they look really skimpy to me and not high quality. Pleats are made much bigger these days and more full, too.

I'd just add to the top to get rid of the pleats and either just hang them from rings, sew tucks at the top to insert drapery pins or just make a rodpockets without a header. Then just add new lining and throw away the old lining.

Hey shadylady, that's a good point... the pleats do look puny now that you mention it. Hmm, perhaps I should just cut the top off and use the bottoms. Each curtain panel is 60 inches wide by 64 inches tall.

This gives me more options - if I use the cream on the bottom and have a different color on top, and line up the fabric transition to match the window sill, I would only use ~ 15 inches of the length, leaving 49"x60" remnants that I could use to make matching faux roman shade valances for the other two windows in the room, or pillows (etc etc)...

I'm just going to add that's it's more work to redo them than to just start from scratch. You'll also have to take out the side hems to add new lining. Just adding to the lining will look bad. By the time you do everything, you might as well have made them new. Just something to think about.

Sorry, I wrote before I saw your last reply. So you're thinking of using the curtain fabric just at the bottom? That would work too.

Are the other two windows shorter?

The other two windows are a bit shorter (54") but I don't plan to make fully functional roman shades for them since they are so close to the wall (not room for a curtain panel). I was thinking of valances/faux roman shades that are fixed in place (I don't need functional roman shade because I will have cellular shades on them - I live in a cold climate and need it for insulation). I was thinking something like this:

I may just be making too much work for myself in trying to reuse these :)

49" should be enough for faux relaxed romans. Altho you may have less if you need to cut away any of the fabric where the pleats are. There may be some discoloring or fading there but may be okay.

When you add fabric at the bottom, instead of just having a seam showing, make a small pleat with the bottom of the old curtain. The new fabric seam will be under that pleat. It's a nice, custom dressmaker touch and will serve you well.

Be aware that when you take apart an established (older) pece of fabric, there may hem and stitch lines that cannot be ironed away. As others have mentioned, this project will be time consuming.