Hello, I need to help and advice on replacing 'Conbraco 36C-504 Water Pressure Reducing Valve' - shown in attached picture.

At local lowes and homedepot, I could not find the same replacement Valve (make and model), Instead they had similar valve on WATTS brand - LFN45 Copper 3/4-in Fnpt Pressure Reducing Valve - Model LFN45BM1-U
(https://www.lowes.com/pd/Watts-3-4-in-Brass-Female-In-Line-Water-Pressure-Valve/1001331056) .

I have decided to purchase this WATTS brand and tried to replace, was unsuccessful. The union tail piece - is not able to fully thread over the valve's threaded edge on both end. The Union bolt does not thread over. Due to different brand the thread is not same.
So I was thinking to replace Union Trail bolt and enge with the one comes with new valve.
I need ideas and help removing the Union attached to the PVC pipe and bolt (shown with arrow in the picture).

Or help me with repair kit for the original valve.
Or any solution would help for my situation.

Thanks