You're aware that contact with their dad is not for their dad's benefit, but for their benefit, right? To keep that relationship strong?
I moved to London about 3 yrs ago, from another part of England. My dcs' dad is still in the area we used to live, as we were separated (now divorced). Long before I moved (well over 6 months), I sat down with my ex and discussed all the pros and cons with him and asked him if he was alright with it. For our situation, the move was to benefit our dcs, and ex was in agreement that it was workable.
As an American, I could have moved with the dcs to the US (and have an open opportunity to do so), but my dcs would have been very upset at being that far away from their dad, and it would have made visits impossible for him. So it's out of the question.
I'll be honest and point out that ex and I have not had the greatest relationship after we separated, but we've finally come out the other side (we've been separated/divorced for 7 yrs now) and we can work together for what's best for our dcs - even if sometimes we drive each other nuts. I work with him on stuff because it's what is best for our dcs.
If you know your ex will not be happy with this, you might want to think about that. Look at your pros and cons - who are they for?