my royal went straight in her 3ft viv and i have never had any problems with her. she has only refused once in the 3 years i have had her and she was a baby when i got her.
Its fine to use RUBs if thats the way you want to go but if you are looking to move to a viv then I would advise to just buy a 3ft one when you are ready to put the snake in it and the snake will be fine as long as you have left the viv for a while to regulate the temps etc before moving the snake.
I wouldn't change the viv sizes as that will get very costly and is not really necesary. A 3ft viv is more than enough for a royal.
Hope this helps and good luck with the snake!