Thanks to Lukas Lang and kglr the answer is as follows:
DownValues@System`GeoPlotsDump`iGeoRegionValuePlot =
DownValues@System`GeoPlotsDump`iGeoRegionValuePlot /.
HoldPattern@
Rescale[#,
minMax : {0, 1} | PatternSequence[], {#2[[1]], Plus @@ #2}] :>
Rescale[#, minMax, #2];
cf = Which[70 < #, Pink, 65 < # <= 70, Blue, 60 < # <= 65, Green,
55 < # <= 60, Gray, 50 < # <= 55, Orange] &;
GeoRegionValuePlot[
EntityClass["Country", "Africa"] -> "LifeExpectancy",
ColorFunctionScaling -> False, ColorFunction -> cf,
ImageSize -> 350, MissingStyle -> Red,
GeoLabels -> ((Tooltip[#1, Row[{#2, Spacer[5], #4}]] &)),
PlotLegends -> Automatic] /. _BarLegend -> BarLegend[{cf, {50, 75}}]