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For a reissued ticket (i.e. not a revalidated ticket), a totally new ticket will be issued. If the ticket is part of a booklet (i.e., there are more than four flights), all the conjoined tickets in the booklet will be reissued as well.
A ticket number cannot be re-used, they are globally unique, and the number is used for accounting and auditing purposes. In a revalidation, certain fields in a ticket can be altered, but that is different to a reissue. In the old days a revalidation was done with a sticker that covered up certain parts of the old ticket; the same limitations are applied to electronic ticketing.
Almost always the PNR will be unchanged, but that is not required. The ticket and PNR are unrelated concepts.