Like the title says, in this instance, Celerity I is required for Celerity II. Selecting to replace Celerity I also removes the option for taking Celerity II (as intended)

Meanwhile replacing Celerity I and taking say, Iron Durability I does not show Iron Durability II as a possible choice. I would guess that because the advancement isn't fully done, Iron Durability I isnt technically a fulfilled prereq for choosing Iron Durability II.