I have not priced it out but I wonder which way would be cheaper? Buying the Ultra and converting it down to a Custom, or buying a CVO Custom or Street Glide and adding some of the Ultra goodies. For me, I love my '13 RGC but its time to go to the geezer side of the house. If I need to, I'll remove the tour pack on my new ultra and turn it into a custom for around town riding.
Lots of us have done the up-conversion of our SESG with Detachable Tour Pak, passenger footboards, etc. Some even go with the speaker pods, etc. I didn't do those because my SESG already has the saddlebag speakers. I spent about $2500 for the TP/mounts/spoiler/LED lighting, liners, etc. An Ultra LED light bar is another several hundred, and a top rack is a couple hundred more. I didn't do either one of those.
I did spend a whopping $600 on passenger footboards and Rumble inserts to match... and I didn't even get the adjustable footboards. They are a hundred or two more, I think. That's a lot of money for friggin' floorboards!
And then decent two-up seats, etc! Now your SERGC/SESG is pretty much at the same total investment as an Ultra... but you still don't have all of the Ultra goodies like heated grips and seat, passenger intercom, etc. I don't need all that stuff, and I just love the SESG look, so I went with an SESG and added the stuff I wanted.
Others have stripped their SEUC/SERGU/SEL with detachable TP mounts, antenna relocator, amp relocation, etc. That can many hundreds. To get the SERGC/SESG look, still others have done the stretched bags and SERGC/SESG rear fascia... now you're into several THOUSAND for that! Add a sleek seat for when the TP is off, and that can be another $500... or much more depending on what you get.
So, if you want the SERGC/SESG look, I'd go with one and add the TP, lights, speaker pods if you want, rack if you want, passenger boards, heated grips if you want, etc... and for SURE a better two-up long haul seat if you do long trips. The SERGC/SESG stock seats look cool, but are complete crap to sit on for hours!
If you want all of the Ultra creature comforts, and don't really care about the SERGC/SESG bags and rear fascia look, then it would be far cheaper to fit a SEUC/SERGU/SEL with a removable TP and a sleek seat. You could do that AND even put much better rear shocks on it for less than adding the TP, boards, good touring seat, etc. to a SERGC/SESG.
If you want both the SERGC/SESG look AND a full-bore touring rig with all the Ultra bells and whistles... it can be done, and has been. Just start with a SEUC/SERGU/SEL to get all of the Ultra goodies... AND THEN OPEN YOUR WALLET REALLY WIDE!
Ken