Change terrain size without changing painting texture

Hello, I need a little bit of help. I have a terrain that I painted myself and it took many days to finish it. Now after some work, I decided that maybe expanding my terrain a little bit can give to my game some good results, however when I change the width and height size my painting texture stretches with the terrain, I don't want that to happen and have to paint again everything. I am really happy with the results in this moment.

Any ideas how can I achieve to make the terrain bigger and the painting stays like it is in this moment

Thanks in advance Sorry for my bad english