Highly
likely:
SBS892T
(HG SP) 
(HG 85)
SBS2671B
(HG 85) (HG SP)
892T has been stuck on Sv 85 for about 8 weeks already
(2months). AM Peak it'll reach Yishun around 0710hrs, PM Peak it'll leave
Yishun Int at ard 1805hrs. Same timings for the entire
period.
And it's definitely not replacing 2678H because 2678H is usually
the bus behind 892T during AM Peak & is still on its usual
Premium slot. 2760C is still on Sv 85 as well.
2671B on the other hand has been on Sv 43 109 107 87 lately and MIA
from Sv 85~
It might seem weird at first at this arrangement. But 2671B
isn't extended yet (and at current is due to go by Feb 2014 before
extension). while the fully-extended 892T is due to go only by Aug
2014. so if 2671B doesn't get extended in the next few weeks, 892T
might be the logical replacement.