I am not a proffessional, BUT! To convert from 3/4 , to 1/2" , you can replace the riser, and use an adapter. They sell risers that screw into the 3/4" to 1/2". Pictured below. You cut them to length with cutters.
I'm no expert..........YET! I just like to suggest things and learn... See what the pro's have to say first.
To convert from a 3/4 threaded male to 1/2 , the proper way would be a 3/4 Thread x Thread Coupler, 3/4 x 1/2 Thread x Thread bushing, and a 1/2 closed threaded nipple.
However I generally go back to the source and reduce down from there instead of all of those fittings at the sprinkler head.
My watering needs in a particular spot have changed due to a new paver patio/walkway, so I'm looking to water two new flower beds along side the walkway. I could also change to a different nozzle on the I20 if the conversion is too complicated.