you can save your project as 5.3 and open it with 5.4 5.5 5.6 XD
usually is everything okay but if you are using a plugin that is not updated you cant use that.
Creazy example, i m using water plugin in 5.5… in unreal 5.6 this plugin still have to be made.
your project in 5.6 cant load the water.
Beside this you should be fine moving also from PC to MAc and viceversa, i never did but a friend do it all the time, and usually fix everything