Technically, a fixed bridge can be removed and replaced by an implant. However, if you already have a bridge in place, why have it removed. The only advantage of an implant over a bridge is that the adjacent teeth need not be involved to give an implant. In a situation where a bridge is already in place the adjacent teeth have been involved and it doesn't make good treatment planning to remove it and get implants. A bridge may or may not affect fixed orthodontic treatment depending on the type of problem, number of units in the bridge, location of the bridge etc. Only an objective clinical assessment will help us decide.