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
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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