Hello Dan58,
Welcome to the community!
Whether or not you would keep or lose that credit would depend on the particular details of your situation. For example, if you received that bill credit towards financing a device, you would likely be able to keep that credit (assuming the new plan is of the same category as your current plan; see here). However, it would no longer be applicable after the 24 months. On the other hand, if it is a loyalty bill credit, it may or may not be compatible on new plans. Your best option would be to contact customer service so they may look into your particular circumstances. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
