No and yes
No, a character cannot replace his backstory. It already happened, so it really wouldn't make sense for them to lose the benefit.
Yes, there are no background ways to get most of these things. This is what the Faction Agent gets you, according to SCAG 147-8:
- Skill Skills (two of your choice)
- Languages (two of your choice)
- Equipment: badge or emblem of your faction, a copy of a seminal faction text (or a code book for a covert faction), a set of common clothing and a bag containing 15 gp.
- Feature: Safe Haven: has access to a secret network of supporters and operatives, etc., etc.
- Features and motivations: Acolyte background, with exchange of & # 39; faith & # 39; by & # 39; faction & # 39 ;.
The first two are not very distinctive. The equipment section could literally be Bregan giving his charlatan a backpack full of things. Safe Haven is he promising some underground support. Traits and motivations depend mainly on the player and are beyond your reach.
So basically, all this could be free. However, as a DM, you will not want to show favoritism, and you may want to provide rule support.
PHB 187, in the Downtime Activities section, is a Training subsection. With an instructor, 250 days and 250 gp, you can learn a new language (or a set of tools).
DMG 227 begins a section called Other Rewards. Near the end on page 231 there is a section called Training, which offers competence in skills as an option.
For a faction emblem, flip a page back and consider the Prestige Mark option of a medal. This should cover the special item of the team.
Finally, for Safe Haven, see the Special Favors section. This could provide a particularly good role play as faction agents begin to come to you to get the favors due or to look for new ones.