I noticed the doctor was wearing his blue suit again in the new trailer.

Now he normally wears the brown suit and his duplicate last wore the blue suit in Journey's End and was left in alternative earth wearing it.

Any significance to change of suit-alternative doctor maybe-or has he bought these blue suits at Suits for You where it's buy one and get the other reduced

Given the size of the TARDIS wardrobe I would imagine it's a 2 for 1 job. Especially considering the situations he finds himself in - no doubt he needs more than one of everything while he gets the alien slime/dalek dust/sand out of the first one!

He also wore it in the Music of the Spheres short for the proms
so he must have more than one

And here I was thinking we wouldn't have to suffer the blue suit again

We've already seen the Doctor wearing the Blue Suit post Journeys End in "Music of the Spheres".

Ok, probably not considered canon but if the Blue Suit was meant to be some big reveal for the return of Doctor 10b then they would not have had the real Doctor wearing it in MotS.