H1B 10.1后继续pending FICA tax 怎么算?

如题，
file H1B as Change of Status
10.1已经过还在rfe, 请问10.1到Change of Status之间FICA tax 怎么算? H1B 生效日怎么算？
律师我现在出境就是

forfeiting your Change of Status and your H-1B petition will become a Consular petition. Once your case is approved you will have to travel out of the United States again to activate your Consular H-1B petition

然后跟payroll沟通payroll回复

Assuming the H-1B petition is approved, it will reflect a retroactive effective date of 10/1/2018 --thus Payroll will be asked to w/hold the FICA tax retroactively from 10/1.

所以是不是不管几号批准， FICA tax 都要retroactively from 10/1？ 哪怕是Consular petition？  

email了律师在等回复。。。如果是怎么都要从10.1开始付就不要找payroll update了。。。

小弟查了很多可是还没有看到相关的解释，地里有没有有经验的前辈？