Cancelling a mobile internet line *after* using the iPad 10th Gen for $99 add a line promo (Offer ID P911)

My brother has an iPad 9th Gen on a $20 per month Unlimited Tablet line on our account. His iPad is one that someone gave him, is fully paid for, and was not purchased through T-Mobile. He said he doesn't need a new iPad, but I was thinking of getting one for my daughter.

I saw that if we add a line to our account, I can get the 64GB iPad 10th Gen for $99. My daughter is young and will just use it at the house, so she doesn't really need mobile internet. yet. I was thinking I could add a new line to get the discounted iPad, cancel my brother's line, and then give him the SIM card from my daughter's new iPad.

The promo is offer ID P911, and I saw in the information for the promo that it says this...

Qualifying credit & new tablet line ($25 or higher) required. If you have cancelled mobile internet lines in past 90 days, you may need to reactivate them first.

The $25 requirement means that I would have to make the new line an Unlimited Tablet w/15GB hotspot plan rather than the $20 Unlimited Tablet plan that my brother currently has. My brother said that is fine with that, though, as he would like to have some mobile hotspot data, anyway.

The bigger concern is the mention of cancelling mobile internet lines. The wording only mentions if we "have cancelled mobile internet lines in the past 90 days" but doesn't mention anything about cancelling in the future.

Does anyone know if I buy the iPad on a new line for $99, will I lose the monthly promo credit if I then cancel a preexisting separate mobile internet line afterward? Or do I have to wait 90 days to cancel?