When a plain Ethernet frame is transformed into 802.1Q-tagged frame the original Ethertype field does not get overwritten. Your original presumption -- that when "802.1q is created Ethertype field is changed to 0x8100" -- is incorrect.
The 802.1Q standard uses tags to mutate the original frame. When a plain Ethernet frame is transformed into 802.1Q a special 32-bit field (tag) is inserted between the source MAC address and the original Ethertype field. So the original Ethertype field is not lost but is shifted right by 32-bits of the 802.1Q tag.
And the 802.1Q tag is formatted in such a way that its first 16-bit field carries the same type of information as the original Ethernet Ethertype field. Though it is actually called TPID (Tag protocol identifier) field. This field is located at the exact same position as the Ethertype field in plain Ethernet frames, and is thus used to distinguish the frame from untagged frames. So when an interested party parses the Ethernet frame header and stumbles upon 0x8100 value in the supposedly Ethertype field it knows that this isn't actually the Ethertype field, but rather an 802.1Q's TPID field of the 802.1Q-tagged frame. And to get to the actual Ethertype value the parser will need to ignore the just read 16-bit field and the following 16-bits (32-bits total of the 802.1Q tag).