I would arm myself with a lot of patience, listen quietly and allow them to vent. Understandably, they are upset ....whether you realize it or not, they feel that their entire world will be turned upside down by your absence. Depending on their age, they may also feel they're being robbed of what little time they have left to enjoy their grandson.
The holidays is also not the best time for bad news. Obviously their grief is so great, they have forgotten or no longer care how they express it. They may even ignore that it's your life and that the decision is yours and your H's to make, not theirs.
Perhaps your H could have a quiet talk with them. Right now what they need most of all is sympathy, understanding and a lot of reassurance. Finances permitting, you and H might offer them the plane tickets so they can come and visit you once you're settled. You could also arrange to come home once or twice a year. If they have that to look forward to, it may ease the transition. Time heals and they will get over it one way or another. Handle this with sensitivity and compassion and there shouldn't be any lasting scars.
I do wish you all the best.
Nervous about grandchild's birth - it freaks me out
Retired husband is driving me crazy - not depressed, just lazy
Beloved dog put to sleep - dealing with pet loss