2016_05_02 After staring at it all day,
I decided to risk pulling it off wall. The latex glue was holding it firmly, but it allowed me to remove a third of the sheet, and probably I could have removed the whole sheet.
The result was that I got the sheet to sit down again. Did not continue, because it is a two person job to get it back up properly, I think.
Seems it might be fixable, which is good news.
Wonder if the latex should be let sit a while, before fixing the paper on the wall. It appeared to go back on well, without extra glue.