If you look at the definition of TA it is the altitude at or below which your vertical position of an aircraft is controlled by reference to altitude.
So if cleared to TA you remain QNH; once cleared above you should set std. You may set std as soon as the clearance to a FL is received (some company SOP's may differ but the law says you may). In decent you again can set qnh as soon as a clearance to an altitude is received. If ATC require FL passing reports you can delay changing your subscale.