Question about upgrade for someone not eligible for upgrade.

We have 4 family members on our boost account. We have had boost for a long time. I needed a new phone but was not eligible for an upgrade. So I purchased one under my moms number. If I just activate the phone with her sim card then switch everything back over will that work? I noticed the disclaimer that the phone had to be for the selected line after I placed the order. Curious if anyone else has tried this. Thanks!