ESS/MSS - Impact of upgrade from 4.6 C to ECC 6

Hi,

I have SAp R/3 4.6C with ESS and MSS. There is plan to upgrade to ECC 6. Want to know if upgrade has any impact on already implemented ESS. Heard from someone that after upgrading to ECC 6, we need to re-implement ESS. It it true.. Can someone pl give more details

Regards

Giriraj