1. PI 7.4 with SAP B2BAddon = PO 7.4 - BPM - BRM . Is it correct ? I assume that we will get separate license for BPM and BRM with PO 7.4 .
SAP PO 7.4 will have BPM and BRM in single SID. No separate license. All in one.
2. What would be the best approach to move from PI 7.4 to PO 7.4(to get SAP B2Baddon functionality) ? Migration or upgrade ?
Migration should help you. Anyways if you have CCBPM scenarios upgrade is needed.
Any specific reason for moving to singe stack/
3. How much efforts for the best approach in months? Considering 10 interfaces already in production environment .
Since only 10 interfaces are there it should not take much time.
4. Risk involved in the best approach ?
No Risk.
You can refer below links for more details