Returning Int64.Type from a function in M

Table.ReplaceValue(
    myTable, 20170701,
    Int64.From(Text.From(year - 1) & "0701"), 
    (x, old, new) as Int64.Type => new, 
    dateFieldNames
)

Why does this throw "type identifier is invalid" on the Int64.Type value and how to I coerce a return value to this type?

(cross-posted in the Desktop forum)