Character/BOff mods cost energy credits. For a new Ensign/Lieutenant, this may make significant (a hundred or so to a couple thousand, depending on just how much gets changed) alterations beyond his/her budget. This can be mitigated a bit by the one free tailor session per promotion (first one earned when you reach LCDR). Note that BOffs get only one free tailor session period. This means that after the initial mod, if you want to just fix the rank pips to reflect a promotion, you'll have to pay a small gratuity to the tailor.