The immigration rules are still being written, it seems we won’t actually know the full extent of the policy and finer details until we become closer to actually leaving of the EU. What we do know is the rules before the end on the planned transition period.
If you are planning on working in the UK or living in the UK then make sure you arrive before the end of the Transitional period. Freedom of movement and the rules that surround freedom of movement will still apply if you have arrived to the UK before March 29th. EU-Citizens will still benefit from freedom of movement rules during the transitional period. The transitional period is designed to ease the effects on leaving the EU. Currently the transitional period is planned to finish at the end of December 2020 but could be extended, so make sure you arrive before the planned date. You will need to apply for a National Insurance Number if you are planning on working.
You will need to apply for Settle Status if you plan on staying in the UK
If you arrive before the end of the transitional period you will be able to stay in the UK indefinitely. Another benefit would be the ability to bring over your family and partner too. You will need to apply for settle status in order to stay in the UK. The current application is free after the 29th of March 2019. Good news is you have 5 years to apply from the date the transition period ends, which gives you plenty of time!
Home Secretary Sajid Javid described the new rules as “the biggest change to our immigration system in a generation”.
To wrap it up I don’t think anyone can really answer the question accurately at this moment in time. Good news is you don’t have to worry too much until the end of 2020, and it will more likely be extended beyond that.