That has happened to me as well - usually after importing a file from Excel. I don't know the cause (perhaps someone can explain that). But it is easy to make the change. Rather than trying to change the variable type symbol in front of the column on the list, go into "Column Info" (right click on the column) and manually change the data type and/or modeling type there. Provided that the column only contains numbers, this will work even when the option is grayed out when you try to do it the quicker way.