Babitha,
I too registered (and have just taken) the 1.2 Exam, and considered changing to the 1.4 exam.
I quote from the voucher sent to me from sun which
you should have:
"This voucher may be redeemed for any active Sun Educational Services examination (those examinations beginnning with a 310 prefix)". In other words the voucher number is OK for 1.2 and 1.4.
My advice, don't ask the Prometric center, tell them you are changing. If they disagree, cancel your appointment and go elsewhere.
Concerning prometric and the rules, my extremely limited experience of them is poor - no ID asked for before the exam today, sounded like a football match going on outside, no security/cameras/invigilator, left with my notes! So imho I would question wether they really know wether you can change or not.
hth
Jon