Will OSCE installer, automatically remove Microsoft Forefront?

It will not automatically remove Microsoft Forefront anti-virus software.
I would suggest for you to manually uninstall the Microsoft Forefront antivirus and after that, kindly install the OfficeScan anti-virus software.

Actually, when OfficeScan client is installed, it automatically removes the following products first:

1. Authentium(TM) Command AntiVirus for Windows Enterprise 4.9x
2. Computer Associates (CA) eTrust(TM) Antivirus 8.1.655
3. eScan(TM) for Windows 8.0.653.1
4. ESET(TM) NOD32(TM) Antivirus build 3.0.642
5. ESET(TM) NOD32(TM) Antivirus 3.0.667.0
6. Kaspersky(TM) Anti-Virus
7. McAfee(TM) Total Protection
8. McAfee ePolicy Agent
9. McAfee VirusScan Enterprise
10. Norman(TM) Virus Control 5.99.0600
11. Symantec(TM) 11.0.780.1109 Endpoint Protection
12. Symantec 11.0.2010.25 Endpoint Protection MR 2
13. Symantec Endpoint Protection 11.0.3001.2224
14. Symantec Endpoint Protection 11.0.4000.2295
15. Symantec 11.0.4000.2295 Endpoint Protection 64-bit Edition

Hope this info helps.