I was grandfathered in under a policy involving surgery. Would I still be able to reenlist or ask for an extension to my contract?

Since there are no specifics listed by you, the only answer I can give is for you to speak with your Retention NCO as they will have all the answers you need.

Yes you can. HRC cleared you for your MOS. Your waiver stays with you your whole career for your MOS.

Without knowing everything..... Any sort of condition that is already documented in a soldier's record be it an ASVAB score line point exception or some sort of medical condition that was deem acceptable to have the MOS then there should be no reason to impede the soldiers reimbursement or extension but as always be sure to contact your local career counselor to thoroughly verify your record.