Join Date: May 2004
Location: SIN (LEJ once a year)
Programs: SQ, LH, BA, IHG Diamond AMB, HH Gold, SLH Indulged, Accor Gold, Hyatt Discoverist
Posts: 7,739
It would be a 25$ change in my book as they will change the cabin class and collect additional miles etc., but as always YMMV depending on agent you get as they may argue cancel one PNR and then confirm the other so that would be 75$. You need to have all needed miles in the account when you confirm F - assuming the waitlist clears.
Now the other way is to book Biz and then call to waitlist F saver against the same PNR. However, with hold times on the phone allegedly going.into hours of unlucky, I'd take my chance with a 25$ or worst case 75$ fee and get it done online as described by the other poster (waitlist F saver as one PNR, then book J saver as another PNR).