Hi Raj,
Every service pack contains new functionality and bug fixes, all these information you can find it in release notes for each service pack.
All the interfaces will work normally, Only you will get problems if PO 7.4 SP12 Patch 24 got any bugs but if there any bugs SAP will release the new patch immediately.
If you have PI sandbox then first upgrade sandbox and test most critical interfaces, if you happy with the results then upgrade the development, QA and then production.
Regards,
Praveen.