Once a take item is on air you can change the content by sending another write (W/) command to change the data fields, then you need to send a read command (Y/ or Z/) to update the take item on air.
Or after sending the write command just select the take item in the sequence in hit enter to take it again, which will update it on air without having to take it offline.
This is intended functionality. CII is a page based workflow and sending the read commands to update content on air is standard practice.
You also might want to make sure you don't have "Set take item offline when overwritten" enabled in Edit > Preferences > CII.
#XPression