Well, the beyblade released in these year are basically follow a similar order.
March: Starter V2 + Starter S2 + Booster R2 + Booster K2 // Starter gV + Starter kS + Booster bR // Starter wV + Starter zA + Booster eF (the first four of dual was released in April, but it was early April and the other two round came in late March)
April: Starter W2 + Booster D2 // Starter dF + Random Booster tW // Starter bL + Random Booster cR (Random Booster is a special kind of Booster)
May: Booster H2 // Booster aC // Booster hS
June: Booster O2 // Booster kD // Booster aH
July: Starter X2 + Ramboo Y2 // Starter lS + Ranboo mG // Starter rP + Ranboo vL
So we can see their similarities. And if we compare the products released in Augest last two years:
2016: Booster U2 + Triple Booster Set N2
2017: Booster bJ + 3 on 3 Triple Booster Set gZ
So in this case, the bey released in this Augest was also supposed to be a booster just like Unlock Unicorn and Blast Jinnius. But now it is a Starter.
I don't know if you know the Samurai Pegasis in ZeroG. All other Starters in ZeroG has all parts new and each Booster has an old Track and an old Tip. The Track (W105) and the tip (R2F) of Samurai Pegasis were both old parts but finally the Pegasis was a Starter, because it is such an important bey, that is the main character in all previews series.
I think this is pretty the same for bX now. bX is supposed to be a Booster if we look at its released date and the part it has. But Xcalibur is such an important beyblade in Burst series, that it has some special gimmicks that indicate the next generation. And also, Xcalibur was the first boss in the story, and it always come with a special launcher. Therefore, although they didn't change the released date or the part it has, TT upgraded bX to a Starter.