Hussah! 
Yes, after contacting the other members of our party we have successfully transferred to next year, it's a day earlier too!!!
Sat 4th June, instead of Sat 5th June,
return boat is a half hour earlier too.
Strange that the first option is -
I would like to keep my booking as it is, in the hope that current travel restrictions to/from the island are relaxed.
So folks, make sure you click the right option & we'll see you at the TT.
Maybe sooner!!

P.S. I've also just noticed the boat (Mannannannannannannannan) is scheduled to get to Douglas a half hour earlier, sets off the same time.