For a reference scenario I have different annual changes, one is for the period 2017-2020 (let's say 0.5% per year) and another is for the period 2020-2030 (-0.1% per year). Could you please let me know which function should I use for this in "Expression"?

Thank you!

LEAP's Growth() function will allow you to specify multiple growth rates for different time periods. So in a scenario which begins in 2017, you could enter it as:

Taylor