Preparation checks for S/4 HANA upgrade

Hello,

In the S/4 HANA upgrade guides,it is said to perform SI checks & Custom code migration too. These steps are done in S/4 HANA conversion.

Why is SI check or Custom Code migration required in S/4 HANA upgrade? We don't do them in ECC upgrades.

In what phase does SUM actually perform the SI checks ?