If your request is granted, the terms and conditions of your contract are varied to take account of that change.
Unless otherwise agreed, changes to your contract are permanent and you cannot revert to your previous terms and conditions. You should get advice about this from your union representative if you have one, or a legal adviser.
You may want to consider asking for a temporary variation for a fixed time only or, if possible, ask for a right to revert to your old arrangements on giving an agreed amount of notice.
Whatever happens, make sure that what you have agreed is clear and recorded in writing, and that you have thought about how your request impacts on other aspects of your employment terms, such as your take-home pay and holidays.