For sure you can switch from PYW to PayBefore and keep your credit. I've done it before. You can do it over the phone. Some of those bonus credits (like the current 50% bonus) is probably not transferable.
Going to PayAfter is going to be a problem until you use up your credits. Since you have 3 lines you could probably transfer all your credits to one line and keep that one as Pay Before, then switch the other two to PayAfter.
Better Together Savings applies across mix-match PayBefore and PayAfter lines on the same account.
All comments are my own opinion and do not reflect the views of my employer or affiliated groups.