[juvenal]

What time of year is best to split and repot a bamboo that's too big for its present container?

Thanks

[New shoot]

I would go ahead now while it is warm enough for the roots to re-establish.  You might need to saw through the rootball, so be ready for that.

I used to work at a plant nursery where we did this quite often.  Sometimes they drop all their leaves, sometimes not.  Don't panic as they bounce back.  Keep them watered but not soaked while they going again.