Page table address field for uncached page

According to Bryant and O'Hallaron's (somewhat abstracted) model of the page table on Linux-like systems, each page table entry (PTE) has an address field that holds one of three kinds of value: (1) the physical address (actually, the page number) to which the virtual page is mapped; (2) an equivalent location identifier for a long-term storage device -- where the page can be found; or (3) 0, for an unallocated page. Suppose a page has been swapped in from long-term storage to main memory. In this account, the kernel replaces the long-term storage address (2) with the physical address (1). Now suppose that same page in physical memory needs to be evicted to make room for some other page to be swapped in. How does the original long-term storage address get restored to the PTE, if it has been overwritten with the physical address? Thanks.