Will this be available for buyers that had only the option for the 750w? My main issue is that I hope they offer this as a standalone purchase for the R13 buyers that didn't get this option.* Nope. The Aurora R13/R14 parts page for the 1000W PSU (43YPM Acbel, R61D8 Liteon, RD0G0 Delta) state "For sale = No". This means that the Dell Spare Parts team cannot sell them as a standalone purchase. but as my order entered 'pre production' last night, they are unable to change mine out.* Correct. All you can do is cancel the order through sales and start over.
So, you can now choose the option to get the 750w psu or the 1000w psu with the R13.  Will this be available for buyers that had only the option for the 750w?  
Aurora R13, now has 1000w PSU in build options