A removable TRV head operates the valve body. On a compatible design, releasing only the specified head attachment leaves the water-retaining parts intact. Mistaking a gland or pipe nut for that attachment changes the job.

A head change is appropriate only where the instructions permit it, you have permission and can positively identify the model and attachment. Stop for leakage, damage, uncertainty or a fastener that needs force. A generic promise of “no tools, no water risk” is not enough.