Is it possible to fully adapt the the ABAP code for S/4 HANA before the conversion?

Hi,

I'm planning the ECC ABAP code conversion to S/4 HANA and I would like to clarify a couple of doubts in order to reduce the dual-maintenance period:

1. Is it possible to apply all the mandatory corrections before the system is converted?

I know there are some changes like using the BAPI fields with _LONG sufix that can be only done in the S/4 system but since we don't have any MATNR longer than 40 characters, is this mandatory to have since the 1st day?

Are there other cases like deprecated objects with a replacement available only in the S/4 HANA but not in any ECC version?

2. Is it possible to use Quick-Fixes in the old system? I don't understand why it works only in S/4, I know some Quick/Fixes require the new system, but probably half of them like adding ORDER BY to a SELECT should be available for ECC too.

My goal is to know if I can use quick fixes and if I can have the system ready before the conversion to S/4 HANA to go directly to Test since the first day.