I would like to differ with this point ... I would say that when Microsoft says "All modern WP will get WP10" , its says that all phones ( irrespective of RAM ) which run the next-gen Snapdragon processors will get WP10 ... That includes 435,532,530,535,630,635,638 all devices which run on Snapdragon 200/400 which are considered next-gen when compared to Snapdragon S4 found on 520/620/720 etc ... Even now , many WP8.1 features run only on next gen hardware specific devices ... For ex. Lumia 1020 didnt get the camera updates as 930/1520 got because its processor didnt support it even though 1020 had 2GB RAM ....
So finally it comes down to whether the processor can handle the OS rather than the RAM shortage .