I asked a question previously as to how I could solve Subsurface modifier creating stretchmarks here
User Denis answered this question. He said I should do an unwrapping operation by pressing U > Unwrap and then press F6 to bring up a panel where I could check the box "Use subsurf modifier". The problem is, when i do the Unwrap operation, the UV layout I spent a lot of time making is completely discarded and replaced by a generated one. After that, yes, I can bring up the "Use subsurface modifier" checkbox.
How can I check the box without deleting my UV-layout?
Thanks for reading!