I would go along and check it out.  Most of those ones only use the harness up to 18kgs then you take the harness off and use it with an adult belt.  If it is harnessed up to 36kgs that is brilliant! 
I just did a google search and found one listed on TM and it had "Turns into a booster seat." which makes me think it probably doesn't harness up to 36kgs unfortunately 
 
 
You could always buy one of 
these and use it with your current seat.