I have just booked two uppers outbound using a voucher to upgrade them from prem to upper. I am new to this but each time I phone I get a different answer.

my vouchers expire next year. One person told me that I can upgrade with vouchers as long as it’s booked before expiration. However I cannot book companion ticket if the voucher expires before return date. Then the next said no to even upgrade! Which is it? Is there anything written down? Also some don’t know about adjusting tax when booking the return. Wish the rules were written down for us as it’s seems a lottery who you get.