Anyone used another carrier as a "bridge" to make a Metro number eligible for T-Mobile promos?

My number has been on Metro by T-Mobile for a while, so it's excluded from T-Mobile's switcher device promos.
Idea: port it out to Spectrum Mobile (an eligible carrier), leave it there a few months, then port into T-Mobile on a new line for a device promo.
Has anyone actually done this?
1. Did it work, or did their system still flag the number's Metro history?
2. How long did you wait — is 90 days enough, or do they use a longer window?
3. Did it fail at point of sale, or get approved and then clawed back later?
It's my own number and my own line. Rep answers have been inconsistent, so I'd rather hear from people who've gone through it. Thanks.