The seating position of my leather armchair has slowly dropped over the years in height. It happens doesn't it?

I am thinking about turning it upside down and undoing the underside with a view to firming it up with something. Not sure what with though... padding, boards, expanding foam?
Anyone tried/done this?
Don't need new a sofa or anything as they still look good. Just want to sit at the correct height and no, I don't like sitting on pillows or something as it isn't as comfortable.
[no nonsense until I get some ideas at least, please

]