Strictly a technical answer because the share-retirement process has tax and accounting consequences in the short run. PFE’s CEO is on record that the shares from the ZTS swap will be retired.
The proper way to look at the ZTS swap from PFE’s standpoint is a large cash-free share buyback. As jq mentioned, this will be supplemented by regular cash buybacks to reduce PFE’s share count still further, thereby boosting EPS.