2009 - Do I still have to drain the whole system to replace TRV?

Recently had CH system chemically cleaned and flushed through which was very successful. Following this four TRVs were fitted. One original TRV which was working as it should was turned off by the plumber during the process. However, he managed to turn it off so hard that I had trouble turning it back on and now it appears that it doesn't work. I suspect his turning so hard has made it lock in less than the NO.1 position. Hot water in the pipe below the valve but not in rad. Question - does the system need to be drained down fully again in order to fit a new TRV, or, is there something that can be done to fix the one already in situ? HELP