If you have a mobile pass you just deactivate the pass. Then you activate the pass again on the day you want to travel.
If you have a paper pass you need to make an exchange of passes to change the validity period. Then you need to check if the conditions of your pass allow an exchange. The conditions of your pass you find on the invoice.
As above. But also- as making it start earlier will mean you likely go on very, very busy days between Noel and NOuvAnnee? DO, DO first make very sure you can get RES on those days -this may not be possible, too late, alas. OR find out if you can make trips without needing those pesky RES.
F.e. if you had the idea to visit UK: forget that then-delay till later.