终于找到了

一直在google, 总算找到官方解释了，说只要符合six year limit exemption的要求，无论是什么签证，或者人不在美国，都可以申请新的H1签证。

http://www.nafsa.org/_/File/_/UCSIC_Guidance_H-1B_Beyond_Six_Years.pdf

In sections 106 and 104(c) of AC21, Congress provided exemptions from maximum stay rules for certain H-1B aliens who were being sponsored by employers for permanent residence and were subject to long delays either for government processing or for visa numbers. The relevant subsections emphasize exemption from the maximum admission under INA section 214(g)(4). Congress did not restrict eligibility for additional periods of admission beyond the maximum six years to only requests for extension of stay.
A qualified alien need not be in H-1B status in order to benefit from sections 106 and 104(c) of AC21. The alien may obtain such additional periods of H-1B admission through a petition to change status from another nonimmigrant classification, or through H-1B visa issuance at a U.S. consulate (unless visa exempt) and admission from abroad.