Depends, to some extent, on what the exterior construction consists of. If you have a roof kit that attaches to the wall or is flashed to the wall, you are going to have more "clean up" work to make it look original.

+1. Lots of variables here including the type of windows that exist. You will probably have a hard time finding someone willing to do this job at any price, and if you do, it will probably be more expensive to try and reuse those than to just get a new flat bank of windows to install. I can't tell from the pic if there is some historical relevance to the windows or something like that, but if the thought is cost-savings, I'd reconsider.

post a picture from the exterior..

remove windows save if possible, rebuild front of building, is there is a cantiliver on the foundation.

More work there than just flattening the wall. Matching siding, possible asbestos, framing, etc. I would leave well enough alone and just replace the 3 windows as inserts.

The bay window is causing the house to sag. Either want to put a flat wall with new windows or would replacing it with a whole new bay window cost less?

How is the window causing the house (wall?) to sag. If your house is sagging, you may have foundation issues.

I think it is not window, but water causing a wall to sag…..

Can you post pictures of evidence of the sag.

If that window was original to the house, sagging as of now is an indication of a larger issue. Is the window was added after the fact, then someone didn't structure that window opening properly.

this is the bathroom right above, very slanted , the bay window is slanted but its hard to get a good picture of it. My boyfriend plans on putting jacks in the basement. trying to figure out the my cost effective thing to do for the bay window. ( upstairs bathroom will be completely renovate)

"My boyfriend plans on putting jacks in the basement."

Be careful doing this as it is seldom that simple. You may end up making the structure worse or simply unsafe by doing this incorrectly.

Oooh, your home looks older and has some lovely features like the wood in that bathroom. Older homes can sag and look crooked, so I think getting a professional in here for structural help is necessary.

+1 on MWM's comments about this not being that simple, as well as the earlier comments that the bay is not causing your house to sag. You have some other structural issues here, likely in the foundation. This is a job for a professional.

Definitely not a job for someone inexperienced to try and do. If something is not done right, it could be a big nightmare. It has to be foundation, since the bathroom window is slanted too. Once its repaired, the bathroom, and the bay, should be corrected by the repair. Please call a experienced foundation specialist to look at it.