Normally, stamps remain valid when the sovereign dies or abdicates. So it is possible to use stamps from different reigns. Because the reign tends to come to an end unexpectedly, stamps from the old reign remain in use until a new stamp of the same face value will have been designed, approved, printed and distributed. It is quite common. It is not unheard of that stamps showing the previous sovereign are released after the accession of the new sovereign.
Building on experience in the US when stamp series transition to a new series ... the higher denominations of the old series are slower to get used up. Their stock turns slower. Thus the combination is most often a low value of the new series combined with a high value from the old series.