Parallel Investigator Challenge Scenario rule states that you can upgrade your pair of signatures in between any scenario. For all Roland decks there is no downside to do it on the transition to the final scenario, since fixing Advanced Cover Up does not matter anymore. (Unless the ending mental trauma would drive you insane, and you really care about your investigator's wellbeing post-campaign.)

+4 for signature weapon is amazing. Hell, +2 for one-handed weapon is amazing. And it solves the problem of being at the end of the scenario, collecting all the clues, and not having your gun do very much. Now it has ammo as long as you’re able to keep killing someone, which with enchant weapon, you’re going to get a 3 damage alpha blast at a +7. With quick study, things get even crazier. I’ve built a deck around this card and had no regrets.