Torah Temimah cites Zevachim 5b.
“It is an asham” — an asham that has been reassigned to grazing (owner died / lost and replaced) may be slaughtered as an olah; otherwise it must remain an asham.
Diyuk of “hu” — in its proper state the animal remains an asham.
Only after formal reassignment to grazing may the animal be offered under a different designation. An ordinary asham that has not been diverted must be slaughtered as an asham; slaughter for the sake of an olah would be invalid.