If you start a new play, then it is just like starting the game from ground zero....but you will have access to anything purchased with the real money currency(which used to be called Silver in FC5). If you have a friend who has the game, and their game has weapons unlocked, if you get enough stuff in their game, you can unlock high level weapons in their game and keep them in your game, to have them at a early level....kinda a workaround.....
As far as ways around this, there is only one way I know, and that is available to PC players and it is to copy the save files to a backup location. Thus when you want to restart your old play or character, you copy the new file to another location and move back the original. Which can be a bit tricky since they files are basically named the same, so you have to be careful to not overwrite them when you are making the backups.