Question about $830 discount for new iPhone 15

Hola,

Currently I have a Magenta plan and an iPhone 13.  I want to upgrade to an iPhone 15.  It says that, if I change to the Go5g Plus plan, I can get $830 in credits for a new device (with trade-in)

I just changed my plan to Go5g because I figure the $830 in credits pays for the difference in monthly fees.  But it says my new plan doesn't go into effect until October 27th (one week from today).  Do I have to wait until then in order to take advantage of the $830 discount offer?  I was hoping to buy the new phone today, but it seems that I have to wait until the new plan takes effect.  Or do I?