is there an efficient way to set / change the footprint of all resistors to a specific value?
In Tools -> Assign Footprints you can set one footprint, use the context menu on it, copy it, select several resistors with normal keyboard + mouse combinations (shift and ctrl) and paste the same footprint.
A better way is to use Tools -> Edit Symbol Fields where you can group symbols by reference and set the footprint at once.
My prefered method is to place one resistor on the schematic, then assign a footprint, and then make copies of it for all the other resistors.
This won’t help you now, but it will save you a bit of time for your next project.
With the tools eelik described, changing the footprint links of a lot of resistors is easy. Most of the benefits of the copy method is that it’s a little bit quicker to copy an existing resistor (hover over it and press c) then to add a new resistor from a library (Meh, it’s basically a for add, and then r and [ OK ])
The copy method also helps a bit if you have both horizontal and vertical resistors (with placed RefDes, a.k.a “Reference” in KiCad).