The half dose followed by a full dose regime for the Oxford/Astra Zeneca vaccine hasn't been approved. What has been approved is two full doses but with a longer gap before the second dose. There is evidence that the longer gap improves overall efficiency and from what the expert on BBC was saying this morning the first dose delivers nearly all the benefit.
This may not be the case with the Pfizer vaccine and I would hope that unless the evidence is there then it should be administered as originally planned. We should be going all out with the Oxford vaccine generally anyway as it is going to be available in large quantities and it can be delivered in more or less any location.