phase-in definition by American Heritage Dictionary
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A gradual introduction: a phase-in of new personal policies.
phase-in - Phrases/Idioms
Introduce one stage at a time. For example, New technology must be phased in or the office will be overwhelmed. The antonym is phase out, meaning “to bring or come to an end, one stage at a time,” as in The department is phasing out all the older computers. [Mid-1900s]