HI Kunwar,
To sum it up basically if everything runs fine on BW 7.5 on HANA with your current objects then SAP also recommends to no do any conversion as all the present objects are also HANA optimized.
However the other part is also true that presently there are several properties of existing Infoproviders that the new ones lack, but everything will come slowly with new releases.
1) Like, you cannot use CP for temporal JOINS presently, you still have to use the Infoset.
2) You cannot use JOIN first in CP and then UNION .
3) ADSO's are not basically designed for direct reporting, you cannot enable Nav Attributes in ADSO's.
However all these limitations will be addressed in future releases OR they are the new norm and new designs should be based on these limitations if you are using newer objects.
Hope this gives you some perspective about the limitations
regards
Manpreet