My team rider Richard moved from the 10' Whopper down to the 9'5 at the start of last Summer and is a massive fan of the Whopper Junior.
Richard started on an 11' Robert August and every time I get to see him surf I am absolutely blown away at his Surfing progression.
It seems lots of SUP riders get new or different boards too often.
Richard's progression and timeline of coming down in size was realistic and his current surfing compared to two Summers ago is very impressive.
Skill set, the rider's actual weight, local surf conditions and SUP surfing ambitions may effect the benefits of coming down in size from the 10' to 9'5.
When I was a shortboard fanatic (pre-SUP) my friends and I had a theory that as long as the board had dimensions that suit and were familiar, 'you should always surf better on a new board'.
The progression of board design combined with the progression of your surfing is a natural evolution.
Regards
Jack