I'm afraid it'll be too much of a drop in size, however going down from 700c to 650b/c* is doable depending on the frame, the Surly Pacer for example can be converted from 700c to 650b;
The original max clearance for 700c is 32c (28c with mudguards), adding 650b allowed you to run even bigger tyres says 38c, even 42c (with mudguards).
I'm afraid it'll be too much of a drop in size, however going down from 700c to 650b/c* is doable depending on the frame, the Surly Pacer for example can be converted from 700c to 650b;
The original max clearance for 700c is 32c (28c with mudguards), adding 650b allowed you to run even bigger tyres says 38c, even 42c (with mudguards).