Switching sim card for new locked phone on existing plan

Hi. We have an existing plan with two unlocked phones.

We are considering transfering our number from our VoIP phone to a third (new) cell phone on the plan. Regarding this, I have two questions:

1) Are the current phone deals (discounted prices with a transferred line) applicable for a transfer from a VoIP line, or does it have to be a transfer from another cell phone carrier?

2) If we transfer the number and purchase a new (locked) phone, would we be able to swich phones within the plan such that one of the two existing lines gets the new phone and the transferred line gets one of the old phones? Does that void the discounted price on the new phone? I assume that it is possible to switch phones within the plan (in general). Please correct me if I am wrong. But I want to make sure that we don't void the discounted price.

Thanks!