From Ts and C's on the website:
6.8 Subject to clause 8.2, You may on no more than 4 occasions during the Season transfer Your Season Ticket to a person who is known to You personally for use by that person in connection with a single Home League Match, provided that such person would be entitled (under the Terms and Conditions of Entry) to purchase a Ticket for and attend such Match. If the transferee of Your Season Ticket would have been required to pay a higher age related price for the Season Ticket than You paid, then You or they will be required to pay an upgrade charge of the difference between the Nominal Match Price paid by You and the Nominal Match Price that would have been payable by the transferee. On all subsequent occasions the upgrade fee will be the difference between the Nominal Match Price paid by You and price of a Single Match Ticket that would be payable by the transferee for the relevant Match, rounded up to the nearest whole pound. No refunds would be payable for downgrades from a higher to lower price category or from a dearer to cheaper part of the ground. You are required to notify the Ticket Office on all occasions which You are not using