SkilledMusician
ATC will accommodate the route, levels & destination shown on the FPL - as far as it can. There may be a need to allocate a slightly different track to that requested because of traffic conditions, airspace restrictions, slot time restrictions etc.; & FLS will often be changed because of separation requirements. Otherwise, pilots generally get what they want - standby for examples otherwise !
If the aircraft has to divert en-route that will be dealt with without a new FPL needing to be filed. However, ATC will need to be given details of the new route & destination etc. (over the radio). If a new FPL needs to be filed, for some reason, that can be done by the airline company or, details can be taken on the air (on a "quiet frequency").