Means carrying different size tubes unless you run TL.
You could buy a kit and have the motor laced and trued into your existing rim or simply run the 27.5/650 & 28/700 size.
Main difference is 650 can accept wider tyres if you need them.
I initially purchased for my Carrera Subway 2 fitted with 27.5 wheels a Yose Power 700c rear hub kit and purchased a 700c disc front wheel from Halfords. After running it like this for some time I rebuilt the rear motor into a 27.5 rim with new spokes and refitted the original front wheel as I wanted to run much wider tyres than the 700c rims would allow.