Push them into the opening inside your existing windows to create an insulating pocket of air.
Insert windows for existing vinyl windows.
Replace the entire existing window down to the house frame.
If you need to stick to a tight budget you may want to look into vinyl windows.
Your question if you can use replacement windows would be no.
Fix it with a replacement window to seamlessly match the existing windows in your home.
However before deciding to tackle the how to install replacement windows job yourself check the condition of your existing window frames.
Next measure the width at the top middle and bottom.
Full frame windows and insert windows.
Insert windows preserve the original frame exterior trim exterior siding and interior casing.
Sliding windows slide open horizontally saving space while offering a clean modern design.
Write down the measurements and use the smallest measurements for width and height to buy your new window.
This is a much bigger and more complicated job than installing a vinyl window insert and may require a pro.
Keep the cold air out in the winter.
They will stop drafts dust air infiltration.
Even relatively new vinyl windows less than ten years old can suffer from seal failures and degradation from exposure to weather and may need to be replaced.
Just to get it out there you could use them but only after you remove the vinyl window completely and install them like it was new construction.
To do it that way would be a little counterproductive.
One solution that has been around for a while is the window insert an acrylic window that fits inside your existing windows often held in place with snap.
The window insert to the rescue.
More advanced project because you have to remove interior and exterior trims and sometimes the siding.
Necessary when existing window frames are old and deteriorating.
Insert windows install directly into the existing frame of the window being replaced allowing you to get the latest window performance features while minimizing disruption to your home.
The extrusions of the vinyl windows would not allow that.
You have two choices for vinyl replacement windows.
If they re rotten or badly out of square the whole unit including the frame may need replacing.
Start by measuring inside the existing frame including the height in the middle and left and right sides.