There could be a few explanations melon.
He may just be getting older and sleeping longer. Maybe he is out and about all night? try putting food all over and see if it vanishes overnight.
He may take longer to settle in the new cage and get things to his liking.
I have seen this happen in some hamsters who have been in tank type cages and have been moved to barred and vice versa. The hamster does not feel secure in the more open cage and can become more reclusive. It is quite stressful moving cage for them. Moving the other way, some don't like to be enclosed! We try and keep cage changes to a minimum these days to reduce stress.
They are ungrateful eh! Spending all that money on a hamster heaven and he does not appreciate it :LOL:
I guess you need to decide if you feel he is stressed and want to move him back to the Gabber. A Rex is a fine size for a syrian, even an active one. If he prefers the rex then you will need to get a new hamster for the heaven