The deadline for the transition is 6th April 2021 however the legislation changes are aimed at your end client or the fee payer to determine your IR35 status. Unless your end client has told you already that you are outside of IR35, the probability is that you will be issued with a date and a new contract that firmly puts you inside of IR35, regardless of your working practices. If you want the freedom to choose your umbrella company, start looking now rather than being forced to use one from a pre arranged deal.