I transplant within the first 2 weeks of flower with no apparent harm.
The quick and dirty way is just by turning the pot upside down while supporting the soil with my other hand and smacking the bottom a bunch of times until the whole mess falls out, into my hand of course, then carefullt supporting the root ball and setting it gently into its new larger pot.
But if you have more space than plants and don't need to crop aggressively (keeping plants in the smallest possible container to save precious space), I'd suggest repotting one week before switching to 12/12.