I have asked this question to many staff at harley dealer, haven't got a confident/affirmative answers. 

I want to change my solo seat to 2-up. To do that I need to relocate the seat mounting socket/nut/cap whatever. I need to move them to the last one on at tail. However, the last hole is used to fix the rego plate, aka one more layer under the fender (thicker), does the seat mounting kits still work? I haven't seen any video/photo to prove it works, all youtube video I found are with side mount rego, so they don't use the last fender hole to hold rego plate. 

I am too afraid to try on my bike. I need it everyday for commute, if things fall apart at 11pm, I will pannic. 

Another question is, instead of relocate the mounting kits, can I just install two kits on both locations? so I can swap solo and 2-up seats easily? 

thx. 

But all the Harleys I've owned have been double two up bikes, never had one with solo. having Said that, the thread they use to hold the seat can be easily removed and replaced with a new thread. I would have to look myself but could even get a longer bolt  two nuts and a lock washer , or flat washer. use first nut to secure the rear licence plate then put seat tab on to the same bolt and second nut to hold the seat. Just first of all see if dealer has a seat like you want to use so you can remove your seat and fit the seat style you want to use, just to make sure it fits. fit secure at front push rear of seat down check rear tab hole lines up with the hole. I've even seen ocky straps used.

good luck

Good luck with it mate.

but by doing that, will the seat scratch the paint?