woo hoo! congrats!
I always repot new plants (after blooming, if they were), regardless of time of year. Never had any problems, and I'd really rather not have a plant sitting in possibly very old sphag for several more months.
Just my experience with repotting, anyway - I think at worst, you may slow growth (plant 'pouts') after repot - can't say I have noticed this, but could be happening, and I just haven't noticed
To me, slow growth is preferable than possibly having the plant in old, broken down media ...
Seems new growth usually starts not too long after blooms are done, so I don't think you need to worry about being too late in the season