Pretty much the title. Also, what is the point of such a long "cooldown"? If there is such urgency that needs to be adressed, maybe a confirmational email? A week period WITH and email. I just think this is a shoot at your own feet, i just meant to change my active account from blizzard to ps4...
Please keep in mind that the Migration and the Cross-Save are two different systems. The Migration will happen as long as you have properly linked a Blizzard account and a Steam account over at the PCMove page ( https://www.bungie.net/PCMove ). This will occur whether you have Cross-Save enabled or not. Enabling Cross-Save is not a requirement of the Migration. So, no, you won't lose your Destiny 2 licenses because you have disabled Cross-Save.
Everything you have will still be there. The 90-day cooldown is intentional, as it prevents people from using cross-save to link other people's accounts for what would amount to an account recovery.