What training do you think is necessary?
Well, ALL leaders will need to take something similar to the Venturing coed YPT. Since the BSA tracking system won't know which units are coed, single sex or otherwise, national will need to find some way to track that so that they know who is trained for their position. That would require an update to the system and likely some changes to the back end registration system to record "unit type" regarding coed status.
Presumably, all leaders know what a girl looks like. Seriously though, there will have to be a change to the YP guidelines so that the same rules for coed outings that apply to Venturing are also applied to coed outings at any other level where coed outings will exist. Maybe everybody needs to get a refresher in YP Training to make sure everyone understands this. Okay, so that's 30 minutes, and some people would be renewing the course anyway because of the 2-year cycle. (And this training is already "developed" - they would just have to add one or two sentences that already exist in the Venturing version.) This does not seem like a huge burden, either on the BSA side, or the local volunteer side.
What other training do you have in mind?
You are over simplifying this issue. BSA will either need to make coed YPT mandatory OR they will have to find a way to track the permutations of YPT they currently have.
Oh, and the current Venturing YPT is focused on 14-20 year olds. I suspect there are additional issues dealing with younger girls they will want to integrate in to the new coed training.
Oh, and then they have to re-brand the current coed training so that it is not just Venturing based.
Oh, and now rather than just having two "adults" around you have to have at least one trained male and one trained female adult around. So now you need to recruit these folks to cover your program.
This is much, much more than slapping a label on the current YPT and popping it up on their website.